Auditor-General Lyn Provost says she will look at other examples of ministerial spending "to test the policy" as part of her look into Government ministers' use of credit cards.

Housing Minister Phil Heatley quit the Cabinet last week after admitting he misused his credit card and Ms Provost was asked to look into what happened.

Ms Provost said the inquiry would look at expenses charged to credit cards, expense claims and other related expenditure that provided, or had the potential to provide, private benefit to a minister.

The inquiry is being carried out at the request of Prime Minister John Key, Mr Heatley and the Department of Internal Affairs.

The terms of reference are:

* Audit the relevant expenditure incurred by Mr Heatley's ministerial office under Vote Ministerial Services from when he became a minister in November 2008 to February 25, 2010. This will include examining whether the expenditure was incurred in accordance with rules, policies, and procedures;

* Review the policies and procedures that control these types of expenditure incurred under Vote Ministerial Services to see if they are appropriate and effective. This will include looking at other examples of ministerial expenditure to test policies and procedures. It will also include identifying improvements that can be made;

* Consider any matters the Auditor-General believes relate to, or arise from, these points.

Mr Key said this week that he was relaxed if Ms Provost widened the terms of reference. Originally she was asked only to look at Mr Heatley and the wider procedures at Ministerial Services, but Mr Key said it was up to her to decide what she looked into.

Mr Heatley's resignation followed the release of ministerial credit card statements to the media under the Official Information Act.

It was the first time the information had been released in that format. Ministerial Services is now dealing with requests for the previous Administration's credit card statements.

Mr Key said the information would be released, but he had not asked any questions about Labour's spending, nor had he been told about it.

"My understanding is that it did not bring up any other areas of concern other than ones highlighted and I think there have been satisfactory answers as far as I am concerned ... So I am not concerned about other National ministers."

He also asked journalists to show restraint in their coverage of MPs' and ministers' spending.

"The media do need to think a little bit about what is legitimate spending within the rules, even if they don't like the rules ... If someone breaks the rules they are fair game, if somebody complies with the rules, but the media doesn't like that, that doesn't make that person necessarily a bad person."