The pair were fishing at the Customhouse Quay in Dunedin about 2pm when Mr Aho hooked what they thought was a large salmon.
Mr Amato ran to find a net and returned to continue the struggle.
However, as a crowd gathered and the creature from the deep surfaced, it was revealed to be a shark - at least 1.5m long.
Mr Amato said the pair had started fishing at the quay at 7am and it was the first bite they had all day.
"We heard about people catching 25-pounders [11kg salmon] and that sort of stuff.
"It's our last day off, so I thought we have got to go try it.''
After a five-minute battle, - which drew gasps from the gathered crowd as the shark surfaced, broke loose and returned to the depths.
"I tried to get it up so everyone can see it,'' Mr Amato said.
"But it's too strong. Once it goes down it's done.''
Department of Conservation marine technical adviser Clinton Duffy said the fish was a broadnose sevengill shark and they frequented the harbour regularly.
Despite their sluggish appearance, the sharks could grow up to 3m and could present a threat to humans, he said.
"While they would probably be responsible for more bites on swimmers than any other shark in New Zealand, they were fairly easily dissuaded.''
They could become fairly aggressive and fairly persistent, but often striking them was enough to deter them and as long as they were not disturbed, they would seldom attack.
"It's nothing to be alarmed about. They are very common in the harbour,'' Mr Duffy said.
As with any other shark, people should not disturb them, should not swim if there was blood or burley present in the water and should avoid the water at dawn and dusk, when sharks often fed.
"Normally they are no problem at all,'' he said.
"But every now and again we get one that for some reason will decide to bite someone.''
Broadnose sevengill sharks are identifiable by their flat head and speckled appearance.
Mature males are about 1.5m, although they can grow to 3m.
They frequent the Pacific Ocean and have been found in the waters off China, Japan, Australia, Canada, United States and Chile.
