Details of ministerial credit card spending will be released quarterly to ensure taxpayers' money is being spent properly, Prime Minister John Key said yesterday.

But he does not support opening up the Parliamentary Service, which oversees expenditure by MPs and political parties, to public scrutiny, even though Labour leader Phil Goff supports investigating the issue.

Mr Key said there were a number of reasons not to open up the Parliamentary Service, "primarily being the use of leaders' office budget for things like research".

That could make public potentially sensitive information about party performance, he said.

Mr Key reminded ministers yesterday to be mindful of the high standards expected of them, in the wake of the release of details about ministers' spending last week.

But he said he was comfortable with their spending and some of the criticism had not been warranted.

Trade Minister Tim Groser has been criticised for mini-bar bills, but Mr Key said ministers should be able to have the odd drink on the taxpayer.

"Obviously ministers have the right to have a drink from their mini-bar, and I don't think most people would criticise them for that.

"By and large, what has been demonstrated is that my ministers have been responsible. From what I can see it's been within the rules."

The PM has asked Ministerial Services - which oversees ministers' spending - to look into publishing details each quarter, perhaps at the same time as accommodation and travel expenditure for ministers and MPs is released.

"I'm in no doubt that future requests for credit card usage will be received, so in the interests of greater transparency and accountability, it will be sensible and prudent to formalise this process of release."

Mr Key said Ministerial Services had seemed to accommodate ministers' offices with their spending in the past, and a report from the Auditor-General - expected soon - would clear up exactly what the rules should be.

Yesterday, Mr Goff demoted three Labour MPs for their poor judgment in using their ministerial credit cards, and claimed Mr Key had been too lenient on his ministers; Gerry Brownlee and Phil Heatley remain in Cabinet despite improper spending.

But Mr Key rejected that view because Mr Heatley had been cleared by a report from the Auditor-General.

Mr Goff questioned the wisdom of having ministerial credit cards at all, but Mr Key said it was necessary. If any minister did not feel that way, they were free to cut up their cards, he said.

* Records show people in the Key Government have not been immune to using Crown credit cards for personal items and repaying the money later.
* In March, Maori Affairs Minister Pita Sharples repaid bills of $190 and $80 for hotel expenses, including one night's accommodation.
* In January, a staffer in the office of Civil Defence Minister John Carter put $108 worth of groceries on the plastic, which was later repaid.
* In February, a staffer in the Prime Minister's office used the wrong card, and repaid $71.