The Prime Minister has a level of understanding for Simon Bridges' big domestic travel bill.

The opposition leader has run up a $113,000 ravel bill over the last three months, driving around the country on a political roadshow.

Jacinda Ardern told Mike Hosking she knows how quickly those costs can add up, admitting she was surprised at how high her own travelling expenses had been, when first appointed party leader.

"I remember wanting to want to change my behaviour as a result of that. So these numbers sometimes, when you think you're just out doing your job, you suddenly receive an expense and some of the figures can be extraordinary, it certainly threw me the first time." 

The figures for all MPs are due to be released later this week, but Bridges' was leaked yesterday.

Jacinda Ardern says she has no idea of who or where the source might be.

"Parties have it made available to them at their own expenses, so that's probably a question better asked on the National Party."

Simon Bridges told Newstalk ZB he doesn't know where the leak came from - but he's confident it didn't come from his caucus.

"We'll see what the situation and hopefully the truth will come out over time."

LISTEN TO JACINDA ARDERN TALK WITH MIKE HOSKING ABOVE