Simon Bridges still unconvinced expenses leaker is a National MP

The National party will launch its own secret internal investigation into who leaked Simon Bridges travel expenses.

On Friday, Speaker Trevor Mallard ditched his inquiry, telling National it was an internal matter for them to sort out.

Even though most signs point to the leaker being a National MP, Mr Bridges still isn't convinced.

"I will do my best and the National Party is united in doing its best to get to the bottom of who the leaker is," he says.

A parliamentary inquiry was called off after a text message from the leaker was made public.

The text - which was sent days earlier to Mr Bridges, Mr Mallard and Newshub - asked for the inquiry to be abandoned, citing ongoing mental health issues.

The leaker's text provided three specific details of closed-door National Party caucus meetings, with the author saying "I am in your caucus" and "I can prove this".

Yet Mr Bridges remains stuck on the idea the leaker came from outside his party.

"It may not be a National MP or a National Party staffer," he says.

He wouldn't expand on how his internal investigation would work - possibly because National hasn't yet figured it out.

He left it to Gerry Brownlee to send out a statement, saying: "We only found out on Friday that the Speaker was calling off the inquiry.

"We are now working through the aspects of our own...including funding and terms of reference. These will not be made public."

There was no hint in the statement whether the outcome of National's internal investigation will be made public. Because it's now internal, even if the party does find the person responsible, it may choose to keep it a secret.

The New Zealand public may never find out the identity of the leaker - unless someone was to leak that too.