Simon Bridges calling for a full independent investigation into the source of the leak.

National is calling for an independent investigation to find out who leaked leader Simon Bridges' expense details, with the leader saying he was supremely confident his MPs were not behind it.

Bridges said there would be consequences if it came from within the party, but he expected the results of a forensic investigation to prove otherwise.

Parliament's Speaker Trevor Mallard was considering whether to launch a full forensic review into the release of the details, and he had demanded answers over who had access to the Opposition leader's spending figures.

Bridges said he had spoken with Mallard to convey his preference for an investigation to be carried out independently of Parliament's institutions - it appeared unlikely the Speaker would reject Bridges' request.

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And as party leaders from all sides of the house duck for cover in the fallout of a politically motivated attack, Bridges appears to be calling the bluff of others who have suggested the leak came from within his own ranks.

"I'm incredibly confident it's not going to be a National MP. I have a bit of an understanding and sense of things here, but ultimately I'm confident it's not going to be a National MP," Bridges said.

He said there would be consequences if it was, and he would expect his own caucus to co-operate with an inquiry, which had to be lead by "someone like a High Court judge" and bring in top computer forensic specialists.

"I'm wanting to confirm my very clear view that there needs to be independent, comprehensive investigation. By independent, what I mean is that it has to be away from the Speaker, from Parliamentary Services and from the Government.

"That is no reflection on anyone, I think it's about protecting the reputations in as much as the Speaker is head of our Parliamentary Services and our Parliamentary privileges," Bridges said.

It was important as the leak had implications for the way all MPs carried out their businesses.

THE BLAME GAME

Both Bridges and Prime Minister Jacinda Ardern have swung in behind their caucuses, claiming they did not believe the leak came from either of their sides.

Ardern said only the National Party had access to its MPs travel expenses, implying the leak came from within National Party ranks.

Parties were only given their own expenses to check over before the public quarterly release. However, Parliamentary Service had already confirmed both the Parliamentary Service General Manager and the Speaker had access to all MPs' expenses, and it's understood the Parliamentary Library and others within financial branches may have had access to the full suite of figures.

National has confirmed all of its MPs had the full range of National Party MP expenses.

But Bridges dismissed the Prime Minister's insinuation, saying the Government had been "looking for distractions" following an economic downturn.

"My understanding is there's multiple players throughout the parliamentary precinct, the information library, the Speaker's Office and potentially others as well," Bridges said.

He also defended the travel, saying he was out doing the work the Government had failed to do when it was in Opposition.

CHRISTEL YARDLEY/STUFF National Party leader Simon Bridges has been the target of a political attack, which the Speaker of the House is trying to get to the bottom of.

"I think it's a fundamentally important thing, that as leader of National I am out and around New Zealand - not just in the main centres - and listening to them.

"Both to be a more effective member of the Opposition, but also to prepare for Government. I think what we've seen is they didn't do those hard yards. That is why they are wasting hundreds of millions of dollars on working groups and inquiries right now," Bridges said.

BY THE NUMBERS

The reported figures related to how much Bridges had spent on travel and accommodation since becoming leader.

MPs' expenses were made public on a quarterly basis and Parliamentary Service was due to release all expenses this week. However, Bridges' expenses alone were leaked to Newshub on Monday night, with no comparative figures from other leaders.

STUFF MPs' expenses – what are MPs entitled to?

Most of Bridges' $113,973 in expenses mentioned in the report were spent on travelling the country in a Crown car limousine, afforded to all ministers and the leader of the opposition.

That broke down to $919 a day and coincided with a regional roadshow, which Bridges embarked on to speak to communities around New Zealand at the same time Ardern was away on maternity leave.

Bridges is a Tauranga-based party leader. This would likely mean a higher travel bill on average than his Auckland-based or Wellington-based counterparts.

Henry Cooke / Stuff Speaker of the House Trevor Mallard reveals new details into the leak of Simon Bridges' expenses.

Ardern said she could "categorically rule it out", that the leak came from the Labour Party. The only groups that had access to the information about Bridges' travel expenses were the National Party, and Mallard, she said.

"We've sought assurances from ministerial services, who manage this information, that none of the opposition's numbers were shared with anyone but them. And they've given us that assurance."

KEVIN STENT/STUFF Speaker Trevor Mallard has called for the details of every person who had access to Bridges' spending information.

THE WHODUNNIT

National MP Jami-Lee Ross said he "doubted" the leak came from within National and denied leaking it himself. As a former whip he confirmed that every MP in the party would access to the expenses list, and that MPs from other parties would not.

Ross said he had absolute confidence in his leader and the party was unified.

National MP Nikki Kaye said it was a serious security issue.

"We should be concerned at a security level if this kind of information is getting transferred to a journalist," Kaye said.

She did not believe anyone in National had leaked the figures but conceded she couldn't be "confident" in this.

"I can't be confident, I can just say that I don't think anyone has an interest - National is polling at 45 per cent. I am really proud of the job that Simon Bridges is doing. We are very united."

Senior National MP Judith Collins said if anyone from her party had leaked the figures they wouldn't be in the party for long.

"I think that no one in National would have done it and anyone did then they shouldn't be in National."

MOTIVE AND MEANS

The leak has brought about a round of finger-pointing as operatives from all sides try to cast blame or doubt on others.

If it transpired the leak had sprung from a Government source with prior access to the information, it could spur claims of dirty politics on the left and raise serious questions over the use of official information to attack a political opponent.

Deputy Prime Minister Winston Peters is in a legal battle with former National Party ministers and top level public servants over the leak of his personal superannuation details, during what was widely acknowledge to be an attempt at a political hit job against Peters during the 2017 election campaign.

However, if it came from within National's caucus that could point to unrest within the ranks. But recent polling suggested the National Party caucus was strong - riding high on a consistent figure of 45 per cent, that has not wavered since the 2017 election.

National is still the most popular party, while Bridges' personal figures are low hovering around 10 per cent. He has defended the spend, and says he is confident the leak did not come from within his ranks.

The same sets of polling figures puts Labour's caucus in an equally comfortable position, with a party vote stable in the low-to-mid 40s, however the figures suggest that's entirely off the back of Ardern's personal polling.

Both parties are likely trying to find a circuit-breaker that could swing support their way.