Key Points:

A Labour Party attack advertisement against National Party leader John Key is not a move toward American-style negative campaigning, Prime Minister Helen Clark said today.

The political ad which debuted on television last night uses quotes from Mr Key about his positions on KiwiSaver to make fun of the "two Johns".

The 30 second ad uses his statements to paint him as inconsistent and contradictory and finishes by asking: "Can you really trust John or John in the tough times ahead?"

Watch the ad:

Labour launched its campaign earlier this month saying the election was about trust and the ad will be seen as another way for the party, which continues to make up ground in political polls, of getting that message across to voters.

Miss Clark today told media in Auckland the ads were a way of using humour to highlight Mr Key's "flip-flops" and laughed off suggestions it would be viewed as negative campaigning akin to American-style politics.

The ad bore no resemblance to campaign adverts used in American politics, she said.

While Mr Key today also laughed off the ads, Labour made it clear there would be more to come.

Standing alongside the Prime Minister, Cabinet minister and Labour strategist Pete Hodgson said the ad could have "been a lot worse".

Miss Clark said she believed people would see the funny side of them.

"After all it's all the National Party's own words and they don't say the same thing to a range of audiences. So the flip-flop ads points that out."

Miss Clark did not rule out more ads in a similar vein, saying there was "a rich choice of areas of flip-flops from the National Party to choose from".

Dismissing questions the ads could cause concern among voters that it was using the politics of negative campaigning, Miss Clark said it was about policy and National's "flip-flops".

"It's about the National party saying one thing to one audience, than to another.

"It's about National not being consistent, steadfast, not being resolute; about being prepared to switch its message depending on who it's talking to and it's done in a humorous way."

Miss Clark said it was up to the National Party whether it retaliated or not but Labour was "going to have some fun in this campaign".

Mr Key said while he had not seen the ad he had learned in politics "pretty quickly to laugh at yourself".

"I'm not going to worry too much. If Labour wants to go on a nationwide advertising campaign trying to take the mickey out of me, well so be it."

Mr Key said National would not respond in kind.

"No. We don't need to do that. They can make bigger fools out of themselves without us putting it into a commercial."

The ads are in contrast to the last election campaign when Labour did not mention former National Party leader Don Brash at all.

"They ran out of ideas quite a long time ago and they've certainly run out of money. The only place they have to go is down and dirty," Mr Key said.