Why did Mallard even talk to Ardern about the inquiry?

The Herald reports:

RNZ revealed at 6 am on Friday that the text had been sent the week before, Bridges had a standup at about 9.30 am and revealed he had referred the text to the police, Ardern made her comments about the National Party at about 10 am, Mallard spoke to Simon Bridges at 11.30 am, Mallard spoke to the Prime Minister at 11.45 am, and he issued his statement cancelling the inquiry about 12.45 pm.

Why did Trevor Mallard even talk to the Prime Minister about the inquiry? It has nothing to do with her. It is an inquiry into the leaking of parliamentary information against the leader of the opposition.

Brownlee did not believe there would resistance among the caucus to National conducting its own investigation about the leak with a forensic specialist.

“Most MPs are pretty incensed that the Speaker has gone out and effectively pointed the finger at our caucus and made a couple of pretty serious accusations – one of extreme disloyalty and another of a problematic mental illness.”

Unless the Speaker knows for sure who the leaker is, why has he concluded it is someone within National?