Key's chat with Turnbull caught on the fly

John Key (Getty)
John Key (Getty)

It's something which could cause any man a bit of embarrassment - having your fly down.

It seems Prime Minister John Key isn't immune to that either, and he's blamed it on the Australians.

Mr Key was at Christchurch Airport on Friday when he was spotted by More FM's Simon Barnett and his family.

Wanting to impress his daughter Sophie who didn't believe the pair would know each other, Barnett had a conversation with Mr Key when he noticed something amiss.

The entire incident was detailed on the More FM Breakfast with Si & Gary on Thursday.

"Simon thought he observed your fly was undone, and I said to Simon 'did you tell the Prime Minister?' and he said 'no, because out of the deference to the Prime Minister's office I didn't feel I could tell him'", co-host Gary McCormick told Mr Key.

"I said 'that's ridiculous, the man code says you always say to another bloke excuse me you're at half-mast there or some such thing'."

Mr Key said he would've preferred to have been told, and believed it was a prime ministerial first.

"To be honest, I'm normally careful about these matters because you can imagine the national embarrassment and envy of course that I would create in such a matter.

"It's the first time I can remember it happening as Prime Minister, of course other times, but not as Prime Minister so it was rather embarrassing that I'd forgotten to do that."

He was eventually told by someone, but not until he'd arrived at his next destination.

Mr Key said he didn't "get a lot of time to myself" and got a phone call from Australian Prime Minister Malcolm Turnbull while in the bathroom where they spoke about Helen Clark.

"It's a diplomatic incident caused by the Aussies," Mr Key joked.

And as for whether he'd return the favour, Mr Key said he'd like to think he'd say something to the other man.