A budget estimates hearing on Monday revealed Mr Barnett's chief of staff had run up the highest credit card bill of all ministerial chief of staff over the 12 month period.

Ministerial staff spending grew by 16.6 per cent between March 2018 and March 2019 to a total of over $42,000, the hearing revealed.

On Wednesday, Mr Barnett said the credit card was used to book travel for himself and staff.

"This can include interstate travel and accommodation and can also include expenses for Northern-based staff in Parliament sitting weeks," Mr Barnett said.

Mr Barnett pointed out he had attended a number of commitments interstate during the past 12 months, including building, resources and energy meetings, that would have been booked on the credit card.