Prime Minister John Key is the latest MP to declare his ownership of a racehorse.

Mr Key said he did not believe he was required to disclose his share in the horse Aetherius, but he decided to declare it in Parliament's register of pecuniary interests after being asked about it by TV3.

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"I can't see why I'd need to declare it, but honestly it's so long ago I can't really be bothered going through the arguments so I've declared it."


He said it was not a secret that he part-owned the horse and he had spoken about it publicly on several occasions.

NZ First leader Winston Peters was last week accused of hiding his interest in the successful racehorse Bellazeel.

There was still some debate in Parliament about whether part-ownership of a racehorse was a pecuniary interest.

Mr Peters argued he did not need to disclose his short-team leased share of the horse.

Mr Key said he bought a share of Aetherius with 10 other people as part of a syndicate in 2007.

His share was sold in 2008.

"It would be more correctly referred to as a donkey than a horse," he said.

"I think it managed to win one race where everyone else was running in the other direction, and it now lives in Noumea, hopefully a happy life."

MPs must declare all property, directorships, gifts, shares and other interests in the register each year.

Registrar Sir Maarten Wevers said last week it was up to each MP to decide whether or not something fell within the terms of the register.

He said racehorses that were held by syndicates needed to be declared, but this could depend on the ownership structure.

Independent MP Brendan Horan and National MP Chris Tremain have previously declared interests in a racehorse.