The Dunedin family was walking along the South Otago coastline during a short holiday visit on Saturday when they came across the behemoth.
"We thought it was a dead cow originally, because it kind of looked like the belly of a cow," Julie Costello said.
"But as we got closer, we saw it was this huge fish. I'd be hard-pressed to fit it on my kitchen table."
Mrs Costello said the fish was about 2.6m in length and 2.8m from fin to fin.
It had a missing eye and blood in its mouth, but appeared to be in good condition, she said.
"It's the most exciting thing I've seen on holiday. It was sad, but really cool too."
University of Otago Marine Science Associate Prof Stephen Wing confirmed it was a mola mola (giant ocean sunfish) and said it was relatively rare for them to wash up on Otago beaches.
"This is not something that happens every day, but we get them periodically," he said.
"They are not very common anywhere, but seem pretty dispersed out in the pelagic zone [open sea at a mean depth of between 3.68km and 11km].
"We used to see small ones commonly on the west coast of North America, but one this large is a relatively rare event."
Prof Wing said they were the heaviest known bony fish and average about 1000kg as adults.
Department of Conservation coastal ranger Jim Fyfe said staff took measurements and samples for records, and contacted several museums to see if they wanted the fish.
"Unfortunately, no-one wanted it. So we left it on the beach to let nature take its course."
