National leader Simon Bridges is calling for an "independent comprehensive review" after the leaking of his expense details.
Someone such as a High Court judge would have to lead it, Mr Bridges says.
- Simon Bridges' roadshow cash splash: $113k in taxpayer money on limos and hotels
- Investigation launched into who leaked Simon Bridges' expenses
- Speaker rules out suspects - who's left in the Simon Bridges whodunit?
"It will need to have the best forensic expertise to go through and be prepared and ready and able to go where the evidence takes it in terms of Opposition, Parliamentary Services and also the Government," he told media on Tuesday.
Mr Bridges says he is confident a National MP isn't behind the leak.
On Monday, Newshub was leaked MPs' expenses, which show the National Party leader has spent far more money on travel and accommodation in the past three months than MPs usually manage to chew through.
Travel and accommodation topped $113,973, and most of that - $83,693 - was spent on travelling the country by road and in style.
It breaks down to $919 a day - more than people earning the minimum wage take home in a week.
Mr Bridges acknowledges it's a lot of money, but is unapologetic - saying he hasn't spent too much.
"I'm working incredibly hard as Opposition leader to get out there and understand what's happening in New Zealand. I'm getting out and doing the hard mahi."
But not all Kiwis Newshub spoke to are convinced - and Winston Peters isn't convinced either.
"Is mahi a new Māori word he's learnt? If he means hard yards, he hasn't been doing it," he says