Jami-Lee Ross has been identified as the National Party leaker after the party's inquiry into the leak, but the MP is denying he was responsible.
Watch the full stand-up with Simon Bridges here
In a stand-up this afternoon, the National party leader says the inquiry report identified Mr Ross as the most likely source of the leak, and he accepted that finding.
"The report states that the evidence points to Jami-Lee Ross as being the person who sent the anonymous text message. I am releasing that report today.
"It is in his opinion that on the balance of probabilities the evidence establishes that Jami-Lee Ross was the person who leaked the expenses and the sender of the text message."
Despite the revelations, Simon says he's confident about his caucus and his leadership of the party.
He says these matters were only the result of a single member of parliament and will be dealt with in caucus tomorrow.
"This isn't about me, this is about getting 56 members of parliament, these matters are for caucus to consider.
"The caucus will be asked to consider all relevant matters including his membership of caucus, finally you will recall, Jami-Lee took leave from Parliament given personal health issues, this action is completely separate. I didn't know what the investigation report would contain when the matters were addressed in recent weeks."
But in a series of tweets, Jami-Lee says he's not responsible for the leak.
Later today Simon Bridges is going to attempt to pin his leak inquiry on me. He can not find who the actual leak is, so is attempting to use contact with my local Police area commander, and a journalist that is a friend (not Tova), as evidence that I am somehow involved.— Jami-Lee Ross (@jamileeross) October 14, 2018
The investigation, which was carried out by PwC, was looking into the leak of Mr Bridges' travel expenses in August.
In August, RNZ revealed a person claiming to be a National MP had sent a text message to Mr Bridges and the speaker, Trevor Mallard, pleading for the initial inquiry to be stopped for the sake of their mental health.
That led to Mr Mallard pulling the plug on that inquiry saying it was unlikely the text had been sent from anyone outside the National Party.
The National Party then decided to go ahead with its own investigation into the matter.
All National Party MPs signed a waiver to cover communications dating back to February.
But as the staff's employer, Parliamentary Services refused to give permission on their behalf.
Trevor 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, concluded there was no evidence that Mr Mallard or any Parliamentary Service finance staff were responsible for the leak.