An unusual shark that washed up on Ruakaka Beach last week showed no signs of injury and there was no obvious cause of death, a shark expert says.

A bigeye thresher shark (Alopias superciliosus) washed up on the beach last week, and on Sunday Department of Conservation shark expert Clinton Duffy and Tom Trnski, research manager and curator marine biology at Auckland museum, went to the beach to get some measurements and samples from the carcass to help increase the knowledge about the species.

Mr Duffy said bigeye thresher sharks are quite common but rarely seen due to their offshore habits and he and Dr Trnski wanted to learn more.

Mr Duffy said he and Dr Trnski examined the shark, with help from Andrew Forsythe from Ruakaka Surf Lifesaving Club.

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"The shark was a mature male and was 3.87 metres long, of which 1.83m was the upper lobe of the tail," Mr Duffy said.

"According to Andrew it washed up on Thursday afternoon. There was no obvious cause of death, ie, no sign of a hook, or net marks."

He said very little is known about the species' biology in New Zealand.