By then, it was too late. The three-tonne Ford sort of belly-flopped into the Taieri River, hitting the dirty brown water with a soft splash-thud.
The plunge from the bridge meant a plunge into darkness as the truck nosed into the torrent and water forced its way into the pitch-black cabin.
The reassuring sound of the engine gave way to the sound of rushing water but, in the gloom of the pre-dawn darkness, Mr Spain could see nothing.
For all he knew, he was moments away from becoming the first victim of the 1980 Taieri flood.
It had been raining for the best part of three days before Mr Spain finished the first half of his June 5 morning bread run in the Taieri River.
After a wet end to May, the heaviest rainfall in 57 years was falling on already sodden ground. It helped push the river close to the top of the bridge before washing three 14-metre spans and two piers - about 40m of bridge - downstream.
The Taieri's flow is usually about 50 cumecs. At its peak in 1980 it raged at 2550 cumecs - a quarter more than the largest recorded flood, in 1886, and twice the volume of the recent wet.
Low cloud made it difficult for civil defence personnel to see how bad the flooding might get, but it was soon obvious that the Taieri Plain was slowly but surely becoming a muddy lake punctuated by islands of raised land and the roofs of partially submerged houses.
Mosgiel was awash - the Otago Daily Times reported nearly every homeowner awoke to find their property under water - and Dunedin, assailed by surface flooding and slips in the city and around the peninsula, was cut off by flooding on State Highway 1 and the main trunk railway line.
A state of civil emergency was declared for the West Taieri Plain, between Mosgiel and badly-flooded Henley.
Helicopters, jetboats, tractors, and even dinghies and canoes helped evacuate more than 1400 people from their homes before nightfall. Soon, more than 120 houses would be flooded or isolated.
Farmers, animal welfare officers and countless volunteers moved stock from flooded and flood-threatened paddocks to higher ground. Stock was corralled and fed in safe areas on the by-then closed State Highway 1, and transported to farms as far afield as Southland and Canterbury.
About 7500 cattle and 6200 sheep were eventually evacuated. It would be many months before many of those animals - and their billeted farmers - came home.
When a stopbank on the Waipori River burst on June 7, more than 8000ha of land between Waihola and Allanton, including Dunedin's airport at Momona, was deep under silt and water.
Many areas would stay that way for eight weeks before draining to become heartbreaking, stinking bogs.
The disaster would cost about $8 million - more than $30m if it happened today. Civil defence officials reckoned the plain had never endured such a serious and widespread flood.
The beam from the bread-truck's headlights illuminated some of the flooding on Mr Spain's run from Mosgiel to Outram. A car had slid into a ditch at Wyllies Crossing, and it followed him towards the bridge after he helped push it back on to the road.
When the truck hit the river, Mr Spain (42) felt his way around the partially submerged cabin, looking for a torch, as the driver he rescued moments before sped east through the drizzle to raise the alarm.
His breathing space diminished as the frigid water rose, and he decided he might be safer perched on the roof.
"The cab was going under, so staying there would have been deadly. It was still pretty dark, it was probably sometime between 6.30am and 7am I think, so I didn't realise straight away that the windscreen had been broken and I had a way out," Mr Spain recalled this week.
"I climbed on to the cab, and held on to the radio aerial. It was bitterly cold so I just started flapping my arms around to try and keep warm. I was cold but I wasn't scared - I was much younger then, so I was much fitter and confident - but I was well and truly in the middle of an incredible experience."
That experience had only just begun. The current had already pushed the truck about 1km from the bridge, and the shore was much farther than he could safely swim.
Buoyed by the plastic-bag-wrapped bread, the truck would eventually snag itself near a stand of cabbage trees, about 5km downstream.
"So, there I was, on top of a truck in the middle of a river. I remembered a bit of Morse code from my boy scout days, and I flashed SOS, wondering if there was anyone in the distance who could see me. I was high and safe, but the scene around me was really quite horrific. And I didn't know if anyone knew I was still alive."
MR Spain waited for rescue on a day when police, army, civil defence and fire brigade personnel raced the tide and darkness to herd animals and people from the steadily sinking plain.
Helicopters plucked people from atop flooded cars and power boats raced between flooded properties to ferry people, and sometimes their farm animals and pets, to safety.
Terry Low was one of the men in the Otago Surf Lifesaving Association jet boat that criss-crossed the flooded plain for about a week.
His first job was to help shift aircraft from the flood-threatened airport - later, the boat would motor along a submerged main runway - but the focus soon shifted to evacuating people who had no other way of getting to high ground.
Within four hours, the rising tide covered once-dry roads near Outram and Henley to the point that the stock herded there stood in belly-high water. In one rescue, the boat could not get too close to a man standing waist-deep in water because of low-slung powerlines.
"So we called to him to come to us. He said he couldn't - the water was too deep, he was standing on the roof of his tractor."
Water was up to the windowsills when, on June 6, the crew helped evacuate some of the 15 people stranded in the White House Hotel at Henley. Canoeists would later paddle through the bar.
The boat could not get to everyone. Mr Low manned a helicopter rescue net to rescue a young couple and their three King Charles spaniels at Henley.
He can still remember the sound of the helicopter's rotor blades cutting the branches of the tree beside the house.
"Thinking back, there were so many experiences that made this so memorable. In many places this wasn't just a flood - this was flowing water, going up to 10 knots in some places. It was something unlike anything we'd experienced before."
Trees the size of buses flowed past the bread truck as Mr Spain tried to keep warm. He saw the lights of a fire engine in the distance, and wondered whether he would be rescued before the bitter cold gave him hypothermia. Power lines sparked above him. As long as they stayed where they were, he would be fine.
As Mr Spain flapped his arms and waved his torch, Outram resident Graham Gale - a book-store owner who is now well known as a rescue helicopter pilot - was launching his jet boat from the floodbank at Beaumaris St to take him and Outram Volunteer Fire Brigade Senior Station Officer Les Hyslop to the truck.
Mr Gale's boat had to dodge fast-moving debris to position itself carefully behind the truck to rescue Mr Spain. The freezing cold driver had been off the road for about half an hour.
"It was just amazing to see that boat coming towards me, seeing as how there must have been so many other things that people were dealing with that day. I remember they must've thought I was a bit of a strange one - I'd been out there in the river and the first thing I say to them was something like 'G'day, how are you' or something of that nature."
He was taken to a nearby house where he was given a bath, some fresh clothes, and a bowl of soup to warm himself up. He contacted his wife to let her and their two daughters know he was safe, then went out to help lay sandbags.
"It was all hands to the pump, it had to be. I'd had my scrape. It was time to help out wherever I could."
Alastair Kirk spent June 5 in a mad dash to save up to 50 head of valuable cattle from the flooding on his family's property beside Riverside Rd.
He had already moved his sheep to higher ground, and the water was rising quickly by the time he and his father could start on the cattle.
They tried to drive through the flood but it proved too much for their Land Rover and car. Both vehicles were flooded, so the pair waded through frigid water to get their stock into cattleyards on Murray Rd.
"I can sometimes barely remember what happened last week, but I can remember everything about what happened 30 years ago. The main thing was the cold, the way your legs were so frozen from the water that you felt they were disembodied and numb.
"And we were bloody cold and absolutely wet when we realised we'd have to shelter in the shed because it wasn't safe to go anywhere else."
They pushed against chest-high water to safety. Water rose up the walls as they lifted a dozen or so of the neighbour's pigs to the safety of a wool table. The men were sheltering in the second floor of the shed when a rescue boat arrived.
Mr Kirk was used to flooding at his end of the plain, but he said the volume of water was just incredible. Incredible too was seeing the State Highway flood-free safe havens so full of livestock, emergency personnel, and temporary flooding "refugees".
Mr Spain went back to his bread run the next day, but his truck would have to wait another week before it was removed from near the stand of cabbage trees. By then, the province was starting to count the cost of the worst flooding in memory.
Houses, businesses, and vital infrastructure was damaged or destroyed. Roads were cleared quickly, but Dunedin Airport would stay closed for 52 days.
Nearly 2000 farm animals died, some found in trees.
"It was a horrible thing to happen to so many people. I was lucky, I had my little mishap and then I could move on back to a normal life. Many other people were not so lucky. This flood affected people for many, many months. It was tragic and very sad," Mr Spain said.
Now retired, Mr Spain and wife Margaret spend most of their time driving around New Zealand in their motor home. They are due back in Dunedin soon, for a reunion and their 50th wedding anniversary.
They will celebrate their anniversary at a motorhome rally at a spot not too far from where he sat atop his truck 30 years ago.
And he will look long and hard at the road as he crosses the Outram Bridge.
