Utterly predictably, the National Party leader's inquest into his leaked travel expenses has turned toxic for his party, with the release of the result of an independent investigation. Consultants PwC have concluded the culprit was National MP Jami-Lee Ross, who took sudden leave from Parliament two weeks ago. Ross continues to deny he was the leaker and has responded with some accusations of his own against Simon Bridges.

Revealing he had fallen out with Bridges some months ago over leadership decisions, Ross tweeted: "When I started to become expendable, I confronted him with evidence that I had recorded him discussing with me unlawful activity that he was involved on ... he asked me to do things with election donations that broke the law."

What a mess. Bridges denies the claim and says he is not surprised by Ross' response in the circumstances. Nobody will be surprised. When it became likely Bridges had a disloyal caucus member, his determination to pursue this inquiry was bound to result in recriminations and accusations that would do a political party no good.

Whether the public believes Ross or not - both on his denial of the leak and his claim to have recorded an unlawful suggestion by his leader - the voters can see a party with dissension in its ranks. If the dissension is confined to one MP, the National caucus will need to prove that to the public by dealing decisively with Ross at its meeting today.

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If not, it will be a sign Ross has supporters, or at least sympathisers, in caucus who share his disenchantment with Bridges. Supporters, if there are any, could stand by Ross' continued denial of the leak and challenge PwC's findings. Or they could agree Ross is probably to blame but call it an error of judgment attributable to the personal problems he cited when he sought leave from the House.

However, all of that has become trivial after Ross' response to the PwC conclusions. He has made an accusation of illegality that has to be investigated regardless of what the caucus decides today. In doing so, publicly, Ross has stepped well outside the boundaries of party dissent. Worse, he is threatening to say more about his accusations against Bridges "in coming days". That sounds a threat to the caucus today which would discredit a decision to treat him leniently.

Many in the party must be dismayed that Bridges pursued a minor embarrassment to a point that was bound to become a major embarrassment for the party. What started as a trivial leak of internal travel expenses, three days before they would have been made public, has become a needless test of caucus loyalty to its new leader.

Bridges has left himself and his MPs in a position where anything less than expulsion of Ross - who could not remain MP for Botany if Winston Peters passes his waka jumping bill, which National opposes - will raise doubts that Bridges can survive as leader. It is a mess of his own making.