A shuttle bus bringing cruise ship visitors in to the city was pulling in to a temporary park outside Mazagran, in Upper Moray Place, when it hit the building's veranda about 10.30am on Saturday.
No-one was hurt but pieces of broken glass from a broken bus window went into Mr Nicholson's coffee cup, and the coffee shop table on the footpath was showered with small fragments of wood and paint from the veranda.
"It was quite a big bang.
"The whole building shook," he said.
He remained concerned with safety issues arising from buses parking outside the shop, and noted there had been an earlier incident.
A bus from another firm had also hit the shop veranda, behind the Dunedin Public Art Gallery, while using temporary parking, in a more minor contact the previous Saturday, when the Octagon had also been closed.
Dunedin Passenger Transport branch manager Kayne Baas said the accident had involved one of the firm's buses, which had been subcontracted by a tour operator to help run the shuttle service.
No-one had been hurt.
Mr Baas had gone to the cafe and apologised shortly after the accident.
He claimed that before the accident some street-side parking cones in the area had been removed without authority, making it difficult for the driver to judge the exact location of the shop veranda, which was above the top of his windscreen.
The driver had wanted to park close to the footpath to provide ramp access for two people with wheelchairs.
The bus had received only minor damage, but Mr Baas expected the accident would cost the firm about $10,000 in bus repairs, time off the road, and repairs to the veranda.
The firm had been using the temporary bus parking spaces in Upper Moray Place, specified by the Dunedin City Council, in recent weeks, but a previous arrangement had been preferable.
This had allowed shuttle buses to use bus parks near the Countdown central supermarket, when the Octagon was closed.
Temporary bus parks, including near the cafe, were being used on Saturday while the Octagon was closed off for a parade and events linked to the 150th anniversary of military service in Otago.
Fiona van Leeuwen, the owner of Collectibles, a nearby fashion shop, said a further parking area south of the cafe had also been set aside for temporary bus parking but several cars had parked in those spaces, preventing buses from using them.
More satisfactory temporary bus parking could be provided either outside the nearby Carnegie Centre or outside a nearby car-parking complex, she said.
Dunedin City Council transportation operations manager Graeme Hamilton said the council arranged the temporary parking while the Octagon was closed but was not involved in the running of the shuttle bus service.
The council would also consider whether more parking enforcement was needed to prevent motorists taking parks designated for buses, and whether alternative bus parking sites should be considered, he said.
