Prime Minister John Key after his Post-Budget address at Sky City Hotel in Auckland.

When is a tax not a tax? When the Prime Minister says so.

John Key has denied going back on his word by introducing a "border clearance levy", which will sting travellers with $22 in arrival and departure taxes.

Key deflected questions on the matter from reporters following his annual post-Budget speech.

stuff.co.nz In this exchange with reporters, the Prime Minister deflects questions on the controversial new travel levy.

Asked what the difference was between a tax and a levy, he replied "many", and when pressed, to "Google it".

"There's a lot of technical answers, IRD can give you those answers," Key said. "I could too, but I'm not going to bother."

Tourism operators and airlines have decried the move, saying it was a breach of an agreement struck with the government in 2003.

However, Key said it would not make "a blind bit of difference" to tourist numbers.

"We have to spend more money on biosecurity issues, and those biosecurity issues can be devastating," he said.

"Either the taxpayer pays for it, or the user pays for it. Given half of it's gonna be paid by foreigners coming to New Zealand, I would have thought that'd be an appropriate place to put that."

The new tax comes on top of existing airport charges of around $25, which are already passed on to travellers through ticket prices.

Airlines have confirmed that their fares will rise accordingly to compensate for the new levy.