Simon Bridges has come out swinging against Jacinda Ardern and Trevor Mallard, not convinced the source of the expenses leak is someone within his own party.
Speaker Mr Mallard on Friday called off a parliamentary inquiry into the leak, which saw Mr Bridges' expenses hit the headlines days before they were due to be released.
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Mr Mallard, Mr Bridges and Newshub political editor Tova O'Brien had all received a text from someone claiming to be a member of the National Party caucus, begging for the inquiry to be shut down, citing mental health issues.
The police say they've identified the individual concerned, but won't reveal who it is - and nor will O'Brien.
A fired-up Mr Bridges told The AM Show on Monday it was important for him to know who it was "so the Opposition can do its job, so parliamentary democracy ultimately is safeguarded".
"It may not be a National MP or a National Party staffer. And if it isn't, because the Speaker won't lead the wider investigation - which on Thursday he said he would - we won't get to the bottom of it.
"Nothing's changed since Thursday. Friday afternoon, he came out and said, 'We're not having one.' All that had changed is the Prime Minister had said, 'Oh no, look - this is an internal matter for the National Party.' Well, why? On what evidence? On what basis does she say that? I've seen nothing that's changed.
"If the Prime Minister or Trevor Mallard knows something different to me, please tell me so we can move on with this and focus on real things that matter."
Asked if Mr Mallard shut down the inquiry to protect someone from within the Labour caucus, Mr Bridges said he wasn't going to "make those sort of accusations". But he still wants to know.
"Please tell me - I'm being genuine about that - so we can deal with the serious issues at hand here and move on."
He said while he's "much more focused on the economy, on teachers' strikes and the like", he didn't want to "underplay" the seriousness of the situation.
Watch the full interview above.
The AM Show with Duncan Garner, Amanda Gillies and Mark Richardson, weekdays 6-9am on RadioLIVE and streaming live to the rova app on Android and iPhone.