The Auditor-General is to mainly limit an inquiry into ministerial credit card spending to former government minister Phil Heatley.
However, the spending of other ministers may also come under scrutiny.
The inquiry was launched after the National MP for Whangarei resigned from the Cabinet last week for mis-using his ministerial credit card.
Mr Heatley quit his portfolios in housing and fisheries, after admitting to signing off on an inaccurate expense claim.
Auditor-General Lyn Provost on Tuesday released the terms of reference for her inquiry.
They include an audit of all relevant ministerial spending during Mr Heatley's time in office, dating back to November 2008, to see whether it complied with the rules.
The inquiry will also look at whether the existing rules and policies are appropriate and effective and will test them against examples of other ministers' spending.