It is looking increasingly likely that the person who leaked Simon Bridges' expenses to the media was a National Party MP or staffer.

Photo: RNZ / Rebekah Parsons-King

An inquiry has found no evidence that the speaker Trevor Mallard, his office or Parliamentary Service finance staff had anything to do with leaking the opposition leader's expenses, which significantly narrows down who could be responsible.

Mr Bridges has repeatedly insisted none of his MPs were responsible but now that Mr Mallard has all but cleared his own name, his office staff and the Parliamentary Service staff involved in the preparation of the expenses, the finger of blame is pointing to the National Party.

Mr Mallard initially called a Parliamentary inquiry into the leak but that was overtaken by political events.

His inquiry ended in August after RNZ revealed the person claiming to be both the leaker and a National MP contacted Mr Bridges and Mr Mallard pleading for it to be stopped for the sake of their mental health.

Subsequently, a National Party inquiry was launched - the findings are expected in the next week.

Paula Bennett, who is leading National's inquiry said: "The Speaker's inquiry, which he had publicly stated he was not having, is a matter for him. The National Party is conducting its own inquiry and will comment on that once it has concluded. The inquiry only received the final set of relevant information from Parliamentary Services on Friday."

Mr Mallard arranged a forensic investigation of emails and relevant databases connected to his office and those staff involved in the preparation of the expenses - about 20 staff in total.

KPMG, who carried it out, has concluded there is no evidence that Mr Mallard or any Parliamentary Service finance staff were responsible for the leak.

"On the basis of this independent review there is no evidence that staff in the office of the Speaker, Mr Speaker or Parliamentary Service finance and corporate staff released details of this quarterly expense disclosure report to any unauthorised parties,'' the report said.

As part of the inquiry, key words and phrases were searched which included 'expense claims', 'Simon Bridges', 'financials' and 'National Party'.

Along with emails and printing logs, text messages and calls to or from a certain number believed to be that of the leaker were also collated.

The National Party's own investigation is being led by PWC and Simpson Grierson.

It will consider both the original leak to Newshub and the subsequent text sent by someone citing mental health issues.

The period the investigation is looking at dates back to the time rumours about the National Party leadership under Bill English started circulating - barely two weeks later he was gone.

On 8 February, the caucus gathered in Tauranga for a retreat, which was hosted by Mr Bridges, who went on to become the leader.

PWC will conduct the forensic work and lawyers at Simpson Grierson will be responsible for filtering what information is and is not passed onto Mr Bridges and his deputy, Paula Bennett.

Mr Bridges has given an assurance that no communications between journalists and MPs, outside of the terms of reference of the inquiry, will be passed on to him.