Infighting has broken out in New Zealand’s political opposition, with a member accused of leaking information about his party’s leader firing back with allegations of illegal donation activity.

Centre-right National Party leader Simon Bridges on Monday announced the findings of a long-awaited report into the leaking of his travel expenses earlier this year and pointed the finger at one of his own MPs, Auckland’s Jami-Lee Ross.

“On the balance of probabilities the evidence establishes that Jami-Lee Ross was the person who leaked the expenses,” Mr Bridges said.

“Suspension [from the party] is definitely an option.”

But Mr Ross – who went on medical leave from parliament earlier this month – pre-empted the announcement with a series of tweets denying he was the leaker and saying it was being “pinned” on him because his relationship with Mr Bridges had broken down.

“When I started to become expendable, I confronted [Mr Bridges] with evidence that I had recorded him discussing with me unlawful activity that he was involved in,” Mr Ross posted.

“Working on his instruction, he asked me to do things with election donations that broke the law.”

Mr Bridges vigorously denied those allegations on Monday, saying Mr Ross was just lashing out.

“I reject any allegation of the sort in terms of unlawful activity, it’s simply not true,” he said, later adding it was the actions of one rogue MP.

That Mr Ross “would say those things, given the situation that he is in, I am not surprised frankly by the false comments that he’s making”, Mr Bridges said.

Although the National Party has been steady in the polls since last year’s election, 42-year-old Mr Bridges has struggled to get traction, sitting at 10 per cent in polling for preferred prime minister in August, compared to Prime Minister Jacinda Ardern at 40 per cent.

Ms Ardern on Monday declined to comment on the matter.

National is the largest party in New Zealand’s parliament, holding 56 of 120 seats, compared to the Labour Party’s 46. Mr Bridges was elected leader in February.

While the contents of the original leak – a release of Mr Bridges’ expenses to a reporter early in August – drew little attention, Mr Bridges’ handling of the subsequent saga has been criticised by some commentators.

Mr Ross’ future in the party will be decided at a meeting on Tuesday.