The creature was identified by Auckland War Memorial Museum head of research Dr Tom Trnski and sent to Te Papa in a refrigerated truck.
The 2 metre-plus creature, weighing more than 200kg, was this morning transferred into a permanent 1600-litre tank, so scientists can carry out further studies.
It had been dead only a few hours when it was frozen and sent to the museum, Mr Stewart said.
"We received it here, thawed it and it was duly processed and examined. While it was freshly thawed it was photographed, had parasites removed and internal organs examined."
He said it was wonderful to get an intact specimen in such a good condition.
Te Papa scientists have discovered the sunfish was probably a young male and possibly died from from liver disease.
The fish was "in no way fully grown", with an adult weighing up to 1000kg, Mr Stewart said.
Studies also found the sunfish had eaten jellyfish not long before its death.
The fish was rare and normally lived far offshore, he said.
"In the 30 years or more that I've been at the museum, this is only the second specimen that I've ever seen."
The chemicals that the fish was now preserved in would keep it for "maybe 300 years or more".
It was not known yet if the fish would ever be on display at Te Papa as it would need a costly purpose-built tank.
"There would also be issues around the kind of chemicals used to store the sunfish while it was on display - we can't have that large volume of alcohol in a public space, it would just be too dangerous."
