The McGowan government's payout to senior executives as part of its radical public sector overhaul could leave taxpayers with a $25 million bill, according to state opposition leader Mike Nahan.

In April the state government announced it was slashing the number of department heads, with 20 per cent of jobs in the 521-member Senior Executive Service - WA's highest paid bureaucrats - to go.

So far, Labor has splashed out more than $5 million in payouts for 18 senior executives who have departed the public service. 

Former head of the Department of Training and Workforce Development Ruth Shean got the biggest payout with $424,591.

But with more senior executives liley to go, Dr Nahan has warned the final bill could exceed $25 million.

"There are 21 or 22 who have been moved on from their substantial positions and by the way they have been replaced to a large extent," he told reporters on the steps of Parliament on Tuesday morning.

"So there are no savings there, so eventually it's assumed they will be moved on or find other positions, but they are some of the people who are the highest paid and therefore their payouts will be very large indeed.

"So this is just the beginning and according to the McGowan government a hundred senior executives will exit the senior executive services, which is 20 per cent of the 500 and on a pro rata basis that's $25 million."

The Liberal leader claims the WA government's first budget will be thrown into disarray after the "sacking" or "moving" on of more than 40 senior executives.

He also had a crack at the number of senior ministers, including Premier Mark McGowan, for going on holidays 65 days before the State Budget was handed down.

"Ministers' diaries would normally be filled with Expenditure Review Committee Meetings, meeting with departments and meetings with other ministers to finalise the details of the budget," he said in a statement.

"As former treasurer I am well aware of the work required at this time of the budget cycle and yet the Premier, Treasurer and swag of senior ministers are away on holidays.

"There are Directors General who have been removed, or moved aside from their primary role, who have intimate knowledge of their former department and its expenditure, line item by line item and they are gone."