Watson said he had since cleaned about 500 beaches and roadside spots, and collected a total of 45 to 50 tonnes of rubbish.
He estimated that 85 to 90 per cent of that rubbish was plastic.
At some of his final stops along Ninety Mile Beach late last week, Watson said he was shocked by what he found.
He said he had collected hundreds of items, much of which were microplastics.
"And I found three dead little blue penguins along the short area I walked," Watson said.
"We need to look at what we're doing, our oceans are choking on plastics, a lot of which I feel is coming from land."
"All waterways lead to the ocean, which I believe is the heart of this planet.
"I feel sorry for future generations because what we're doing on this planet is not sustainable. That's what the young will inherit."
Watson began his journey nearly four years ago with a $40,000 inheritance, which he used to set up a truck and trailer he could sleep in.
He left his job, founded Kiwis Clean Aotearoa and on the first day of 2019, got to work.
He had just $17,000 to fund his ambitious mission, so knew he'd needed help.
That's when he set up a fundraising page and invited people to help or support his work with a donation.
"We can all do our own little bit. If you're going for a walk, take a bag and make an effort to collect what you can. It doesn't have to be every day or every week," he said.
"This isn't just the council's problem."
Watson said over the years he'd come up against unexpected challenges, but also heartening support from people of all ages.
"Getting rid of the rubbish has been the hardest thing I've faced," he said.
"When I reach a new region, I contact the council, explain what I'm doing and ask if they can help with the rubbish disposal.
"The answer hasn't always been yes, which was a bit shocking."
His Givealittle page has received more than $54,000 in donations, the latest from someone who'd seen him cleaning Coopers Beach.
In the Mangamukas, he claimed he stumbled upon a "fly-tipping spot" where it appeared rubbish had been illegally dumped for years.
With the help of a few locals, Watson said "460kg of filth was pulled out of the beautiful native bush".
At Long Bay and Te Kouma, Coromandel, he noticed a pattern of neatly cut strips of plastic rope - a byproduct of mussels harvesting.
Watson said he often found waste created by fishing, including heads of pegs from oyster farms, cable ties and fishing line.
"But I'm not having a go at fishermen; my dad was a fisherman."
Among the at least 38,000 kilos of plastic he's collected, Watson explained he'd also found many nurdles - tiny plastic pellets, which are the mostly unknown building block for all plastic products.
Experts agree they cause as much damage as oil spills.
Watson also cleaned roadsides to prevent litter from being washed into stormwater drains and eventually the sea.
"People are still chucking rubbish out car windows. I don't think it's understood that this contaminates our food chain and devastates marine life."
Among the roadside waste, he's found shredded plastic bottles, bits of polystyrene, and rotting plastic sacks - all of which appeared to have been mowed over.
"There's a price to pay for everything isn't there? What about the health and safety of our oceans?"
Some hauls were harder work than others, he said.
At Gentle Annie, near his native Westport, Watson had carried rubbish - including several tyres - kilometres in order to remove it from the coastline.
"It was a massive effort."
Along the foreshore in Wanaka, Watson said he saw two young men collecting rubbish.
He said he'd asked to take their photo to share on his Facebook page, and later donated $50 to each of them for their efforts. He felt he was "just paying it forward".
At the weekend, Watson began his journey south to get some rest and to spend some time with his mum.
He said he remained open to the possibility of continuing his work for planetary health down the track.
"I've tried my best to highlight the problem around our country. And I don't want to make money doing this; I just want to get by."
He now needs to get his truck's brakes serviced, and money for petrol and a ferry ticket.
• To donate to Watson's cause, go to: givealittle.co.nz/cause/kiwis-clean-aotearoa.
- Northern Advocate
