After going behind his boss' back and leaking Covid patient details, disgraced National MP Hamish Walker had only one option.

And that was to resign, says National leader Todd Muller.

"He's done the only thing he could," Muller told Newstalk ZB this morning.

Muller said he had no idea why former party president Michelle Boag sent the patients' details to Walker in the first place. "Maybe the motivations of that might come through the review with Mike Heron QC... we'll see. In terms of why someone in their heart of hearts do these things, only they can answer that."

Walker yesterday announced he wouldn't seek re-election for the Clutha-Southland seat as the party board met to discuss whether to de-select him.

Muller said yesterday: "Ultimately it was a serious error of judgment which cost him his career. His judgment was fundamentally flawed ... and he's paid the ultimate price."

Muller said Walker's decision was "the only thing to do" but said it was "unfortunate" because he'd worked hard for his community.

The leader rejected the suggestion he'd lost control of the National Party.

"Hamish Walker went behind my back and made his own judgment and I'm sure he's ruing it today," Muller said.

Last Thursday night, Walker leaked the details of the then 18 active Covid-19 patients, including their names, dates of birth and which border hotel they tested positive in.