National Party Leader John Key has slammed Labour's blatantly political decision to scuttle the Canada Pension Plan's bid for a stake in Auckland Airport despite the Overseas Investment Office's recommendation that it be approved.

"This sends a terrible message to investors that the Labour Government is prepared to play recklessly with investment rules.

"Labour has put up a 'don't invest here' sign on the New Zealand sharemarket.

"First, Labour surprised everyone with its tightening up of rules on overseas investment in strategic assets – right in the middle of the Auckland Airport process. Now, Ministers have made a decision all in the name of blatant politics.

"Today's decision shows Labour cares nothing about the vital role investment plays in New Zealand's wellbeing.  Helen Clark and Michael Cullen care only about their own electoral survival.

"New Zealanders were told not to invest in housing and were encouraged to invest in KiwiSaver.  But Labour's blatant political interference means it's not safe for savers to put their money in the New Zealand sharemarket.

"The rules are now subject to naked politics, and that means no certainty for anyone.

"Helen Clark and Michael Cullen must explain their startling U-turn on investment.

"When passing the Overseas Investment Act three years ago, Michael Cullen said: 'I want to say quite clearly that it is critical for the future of this country, and for our social and cultural development, that we have a welcoming and open attitude towards inwards foreign direct investment... if this country relied on its own capital resources alone, we would not just not grow; this economy would shrink, because our investment levels would be insufficient to maintain our current level of economic activity.'

"If Labour was looking to encourage investment, why did it shock the market by wiping hundreds of millions of dollars off the value of people's savings in Auckland Airport?  Labour has now abandoned any pretence that investment is vital.  They don't care about anything but their own electoral survival.

"National would have judged this deal against the principle that 51% of shares should be in New Zealand hands in the case of strategic assets.  Having regard to the recommendations of the Overseas Investment Office, if those criteria were fulfilled, the deal would have been allowed to proceed."