Simon Bridges says "it's not the truth" people inside his own party don't want him as leader.

As the investigation into his expenses leak nears its end, The AM Show host Duncan Garner has revealed more from within the National Party.

Garner told viewers and listeners on Monday morning internal polling by the party has Mr Bridges' favourability rating at negative 25 percent.

"This is all nonsense," Mr Bridges responded. "The reality is the National Party is in great shape. We're the most popularly supported political party in New Zealand under my leadership... I feel great about what I'm doing. Every day I get up I'm positive about my job."

Garner said over the weekend he had "contacts" within National telling him Mr Bridges had to go.

"It's not the truth," said Mr Bridges. "The reality is I've got an incredibly strong caucus that is behind [me], we're working hard, we're holding the Government to account... I feel like we're doing a great job in Opposition. We're one year in, we're getting great hits on the Government."

Another party insider revealed to Garner Mr Bridges' election returns were handed in late after amendments were made. Two donations - one of $10,000, another of $14,000 - were initially recorded on Mr Bridges' own return, but removed and placed on the National Party's return instead.

This leak, Mr Bridges accepted was true.

"There's a party and there's candidate - these things were wrongly filed as candidate," said Mr Bridges. "They shouldn't have been, and then they were refiled as party. It's as simple as that."

The Electoral Commission said in a statement there is no penalty for amending and refiling a return after the deadline.

The $10,000 donation came from 'Cathedral Club'. Mr Bridges refused to talk about who that was. According to blog posts by National-aligned pollster David Farrar and far-right blogger Cameron Slater, Cathedral Club appears to be a bimonthly dinner featuring speakers from the political world. Both men imply attendees aren't meant to reveal the details of conversations that take place at such meetings.