MPs racked up a total of $2.1 million on travel and accommodation expenses in the past three months.

However, details for a lot of MPs have not been divulged - and the sum does not include spending by Government ministers either.

There are no surprises given the only outlier, the $113,973 worth of travel and accommodation incurred by National leader Simon Bridges, was leaked to the media earlier this week.

Spending on accommodation and surface and air travel by Labour MPs totalled $494,690 for the April-June 2018 quarter.

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However, the data is incomplete as figures for ministers, including Prime Minister Jacinda Ardern, are released separately by the Department of Internal Affairs.

The department said today the information was not being released at the same time, as is normal practice.

Today's figures reveal a total three-month spend of $2,110,474. This figure is higher than the $1,493,715 spent in the January-March period.

The National Party's MPs racked up $1.4m in the quarter. That includes $26,271 spending by former prime minister Bill English.

Among the Greens, only four of the eight MPs' expenses were included, totalling $80,871.

Of the nine NZ First MPs, the data for four, including leader and Deputy Prime Minister Winston Peters was not included. The remaining five spent $103,613.

Among the relative high spenders on the list is NZ First's Clutha-Southland MP Mark Patterson, who came into Parliament last year. His Wellington accommodation was well above the average at $13,291.

A spokesman for Patterson said the figure was inflated by the inclusion of part of a previous quarter's expenses and backpay for unclaimed allowance.

Parliament's Speaker Trevor Mallard announced on Wednesday an inquiry into who leaked Bridges' expenses to Newshub ahead of its public release.

"It's my view that Members of Parliament and the public have to have certainty that the information they share with their whips and with the Parliamentary Service is cared for with integrity. That is vital to our system," Mallard said.

National MP Gerry Brownlee, Shadow Leader of the House, told reporters the inquiry would almost certainly flush out the leaker.

"Everybody should have a right to communicate freely with their Member of Parliament and if you can have outside bodies accessing that sort of communication, which often is of a very private and personal nature to that individual constituent, then I think you break down the trust that should exist between Members of Parliament and their constituents."