He said a Labour-led stack would lead to at least $16 billion more debt over the next four years - including $6 billion for the superannuation fund.
Commenting on the possibility of having the Greens' support, Mr Key said they had made it "pretty clear that if they hold the balance of power, they're going with Labour".
He repeated his intention to try to sign deals with other parties even if National was able to govern alone.
National's action plan is not for the first 100 days - as at the last election - but for the party's priorities up to the first Budget.
Among the 41 policy pledges cited as priorities in the action plan are:
Economy
* Halve the budget deficit next year, and be back in surplus in 2014-15.
* Connect 58,000 premises to ultra-fast broadband, including 221 schools, by July 2012.
* Establish the Crown Water Investment Company to invest up to $400 million from the Future Investment Fund (FIF) in irrigation and agricultural water storage.
* Introduce legislation to change ACC to allow competition in the work account.
Law and order
* Allow further detention of prisoners assessed as being at imminent risk of serious sexual or violent reoffending.
Welfare
* Reform benefits and focus on getting able beneficiaries back to work sooner.
* Sanction beneficiaries whose recreational use of drugs affects their ability to work.
* Stop the benefit for someone on the run from police.
Education
* Make secondary school performance reports available to parents.
* Set a target of 98 per cent of new entrants to have had early childhood education, to be met by 2015.
Health
* Increase elective surgeries by at least 4000 a year and ensure waiting times are no longer than four months by the end of 2014.
