There is disgust from Judith Collins after the Prime Minister's response to National's plan to double the Serious Fraud Office's investigations budget.

National announced today that the budget would be doubled if elected - to investigate more cases of fraud, bribery and corruption.

The budget was increase to $25 million, and National also wants to rename the office to the Serious Fraud and Anti-corruption Agency

But Labour leader Jacinda Ardern has taken a jab - referring to Collins' resignation from a ministerial post in 2014 due to an engagement with the SFO.

"Interesting to hear now... there's obviously a little history there with the Opposition leader and the SFO."

An inquiry at the time found no evidence Collins acted inappropriately.

National leader Judith Collins told Heather du Plessis-Allan Ardern was wrong as she was cleared for any wrongdoing.

"I just thought 'goodness sake, where's the kindness gone now?' I thought she wanted a clean campaign and that was pretty dirty."

Collins thinks that Ardern must be worried about tomorrow night's debate.

"Gosh, the veil is definitely slipping, isn't it?" 

Ardern declined to appear on ZB Drive.