Deputy Prime Minister Grant Robertson has delivered a merciless but hilarious roasting of the National Party and its recent woes in front of National MPs in Parliament.

Speaking in the general debate at Parliament, Robertson took aim at National for the recent revelations that Todd Muller was forced to resign after his leader Judith Collins learned he had bad-mouthed Harete Hipango to the Newsroom website.

Robertson quoted the NZ Herald's recounting of the late-night caucus meeting last Tuesday at which Muller was told to resign or face suspension. It was, Robertson said, "the night of the short plastic knives".

He said it was the hunt to find out who had "been mean to Harete Hipango."

"Given there were 33 likely, and frankly quite justified, suspects, it was going to be a hard ask."

Robertson called Barbara Kuriger – who had dobbed Muller in to Collins – the "snitch-in-chief."

But he took particular relish in the details about MP Chris Bishop telling Muller to resign with dignity.

"Such is the loyalty and sense of unity in the National Party that Chris Bishop, Todd Muller's numbers man and well-known shiver looking for a spine to run down, was there to kick his mate on his way out the door."

He said National had made a lot about proposed hate speech laws being an infringement on free speech, before also observing National MPs had been told not to talk to the media.

"The biggest threat to free speech in New Zealand is the Leader of the Opposition."

He then pointed out Simon Bridges' tiny revolt: a post on social media featuring Bridges talking to the media.

"I can only see that as a cry of help. So on this side of the House, we are ready to take up the cause. Free Simon Bridges! Free him! Free Simon Bridges from the Collins' cancel culture."

Robertson said Bridges was right to be worried he would be next, recalling Bridges' description of Maureen Pugh as "f***ing useless" (although Robertson opted to use "rather useless").

He then turned to Collins' own past transgressions of her new rule about bad-mouthing colleagues.

"Judith is currently holed up in a floor-to-ceiling glass house."

He recalled Judith Collins saying of Bill English that he was a "poor leader" and unnamed MPs noting it was a bit rich to be lectured by Collins for leaking and backstabbing given the amount of it she had done.

He also mentioned English's description of Collins as "having an unfortunately high estimation of herself".

Robertson noted that as well as an Arctic blast from the south sweeping up the country, National was also now dealing with a northern Arctic blast "from the legal offices of Christopher Finlayson, Esquire."

Former National MP Chris Finlayson had brutally tongue-lashed the party over its woes on Wednesday morning on Stuff.

Robertson described that article as "an empathetic love note to the National Party".

Robertson quoted with great glee Finlayson's comments that National deserved everything that they had come to them, and he had never seen brand destruction as devastating in all of his life.

"It is Chris Finlayson's empathy that I really miss around here," Robertson added.

He wrapped up thus: "Nothing screams alternative Government more than late-night caucus meetings, resignations, scandals, disappearing lounge suites, and MPs being chased through airports. Long may it continue."

Collins had been wise enough to leave the debating chamber well before Robertson started speaking – but most other National MPs were not so lucky.

Their reactions were of pretending to laugh in feigned indifference, and struggling not to laugh - many of them unsuccessfully.