The stakes are high after Trevor Mallard’s announcement of an inquiry into how the opposition leader’s travel costs became public, writes Jane Patterson for RNZ

If a National Party MP is named as the source of the leak of Simon Bridges’ travel bill their political career will come to an abrupt end.

If it is found to be someone employed by Parliamentary Service, they will be marched out of the door.

The stakes are high as the speaker of the house, Trevor Mallard, launches an inquiry into a leak that has all the appearance of a political hatchet job.

It does not look like this was a breach made in error, for example an email being sent to the wrong person.

The Speaker is treating it as a deliberate act with a “forensic” investigation looking at all of the activity carried out on parliament’s servers, that includes any printing or photocopying.

National’s shadow leader of the house, Gerry Brownlee, thinks the perpetrator will “almost certainly” be found and then named accordingly.

The wheels of political reporting are greased by selective leaks but there are two dynamics here that have elevated this into a formal inquiry: the chance, however slim, that a parliamentary staffer – strictly bound by confidentiality – was involved, and National’s active support for an inquiry.

The list of possible suspects has been whittled down and no MP from another party is in the frame.

This greatly increases the likelihood the leak came from the National Party caucus despite vocal denials from the leader and his MPs.

There are still experienced heads in the caucus and they have obviously decided the possibility of flushing out the leak, and the fall out if it is one of their own, is worth the risk of finding out who it is.

If the person is found to be a National MP, that would be clear grounds for expulsion from the caucus.

Running alongside all of this is the passage of the controversial waka jumping law, opposed by National but being pushed by New Zealand First.

It is still before the House but if it is passed before the completion of the inquiry and a National MP is named, that law would ensure any MP expelled from their caucus automatically loses their seat in parliament.

Despite National’s criticisms of the imminent law change, it could end up helping that party because at the moment expelled MPs are free to stay around at least until the next election. The waka jumping law, once in place, would make short work of any MP finding themselves out of favour.

But there is another fundamental issue at play – the protection of journalistic sources.

There was a full blown Privileges investigation after Parliamentary Service gave a press gallery reporter’s phone and swipe card access information to an investigation into the leak of a report into the GCSB.

As a result, Parliamentary Service was given a serve, reminded about the protection of journalistic sources and that consent should always be sought first.

More recently, deputy prime minister Winston Peters named journalists in the initial part of a legal action to find the source of who told media about his superannuation payment.

Mallard assured the journalist who received the Bridges leak this latest inquiry is not about coming after them; Parliamentary Service will have also learnt the lesson of 2014 as this new investigation gets under way.

It holds an extraordinary amount of information about the movements and communications of everyone within the precinct, but there are high expectations similar information would not be handed over without question as happened in 2014.

The speaker does not expect the inquiry into the Bridges leak to take long and, like Gerry Brownlee, expects the culprit to be caught.

If it does turn out to be an MP, hold on for the ride.