His response to criticism of their hefty allowances has been to order a review of the "arcane" regulations and he says he wants changes that will be fair to taxpayers and ministers.
Ministers who have attracted the most attention are those who own homes in Wellington but rent them out while they live in taxpayer-funded houses.
And in Deputy Prime Minister Bill English's case, he claims nearly $1000 a week to live with his wife and children in a million-dollar home owned by a family trust.
Mr Key launched his defence strategy yesterday at his post-cabinet press conference.
"Most New Zealanders, I believe, would support me in my desire to see the marriages of my cabinet ministers and the happiness of their families remain intact," he said.
"I have ministers working 18 hours a day six to seven days a week. They have families and I want those families to stay together."
But he said he wanted new rules because those being used were complicated and led to "perverse outcomes".
Current rules didn't encourage ministers to stay in the homes they owned, he said.
"We need a system which says it's more logical if a minister can potentially stay in their previous accommodation and that doing so might be in the interest of the taxpayer.
"Or if there is any pecuniary benefit, there might be a very strong argument that they get a lower housing accommodation allowance."
Housing Minister Phil Heatley is one of those who owns a home in Wellington, rents it out and claims more than $1000 a week for a ministerial house.
Mr Key said Mr Heatley, who is on a $243,000 salary, owned a two bedroom apartment and couldn't be expected to shift a family of five into it.
And Foreign Minister Murray McCully, who also owns a Wellington apartment and claims about $700 a week for his ministerial home, had to entertain foreign visitors so it was legitimate.
However, some Wellington-based ministers who own their homes either don't qualify for an allowance or don't claim it. They include Attorney-General Chris Finlayson, Revenue Minister Peter Dunne and Consumer Affairs Minister Heather Roy.
