Already under pressure because of the "teapot tape", John Key may have added to his legal woes with a signature.

Yesterday, the grinning Prime Minister walked in Phil Goff and Helen Clark's shoes and autographed a banknote - technically breaking the law.

It is illegal for a person to "wilfully deface, disfigure, or mutilate" any banknote without permission from the Reserve Bank.

Even Sir Edmund Hillary, whose face is on the $5 bill, was not above the law.

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In 1993, the Rotary Club had to go through months of preparations and negotiations with the Reserve Bank to allow Sir Ed to sign 1000 $5 notes for charity.

Cafe Romeo's Romeo Jovanoic is owner of the $10 banknote autographed by Mr Key yesterday.

He told the Herald it was pinned to his fridge in his Upper Hutt restaurant next to a $10 note which Helen Clark signed during her 2008 election campaign.

Defacing a banknote carries a penalty of up to $1000.

But Mr Key is not the only current leader to have signed one. Last week, while campaigning in Nelson, Phil Goff signed a $5 bill for local man Peter Hansen, who said the Labour leader was the 89th public figure to sign a banknote for him.

"I know it's illegal - they've all said it - but the funny thing about it is it's just a good collection that will be worth something. The only person who's ever turned me down was [poet] Sam Hunt," Mr Hansen said.

Auckland criminal barrister Ron Mansfield said a complaint would have to be laid with the Governor of the Reserve Bank or the police before action was taken.

"It's not the most serious of offences, but it is still an offence which would result in a conviction and a fine," Mr Mansfield said.

Last year, Mr Key drew an autographed doodle of the Beehive for an Eketahuna fundraising drive after refusing to sign a banknote.

Organiser Margaret Parsons said at the time: "We wanted him to sign a banknote but he wouldn't because you can't deface notes, so he came up with his own suggestion."