Prime Minister John Key is reiterating he will not stand down ACT party leader and government minister John Banks.
Opposition parties say Mr Key's continued defence of Mr Banks is becoming untenable.
The police found there was insufficient evidence to prosecute Mr Banks for donations from SkyCity and Kim Dotcom during his 2010 mayoral campaign that were not disclosed because they were deemed anonymous.
Police documents from the investigation contain several sworn statements, but the formal statement made by Mr Banks was blanked out.
Mr Key says he takes Mr Banks at his word when he says he did not know Mr Dotcom made a donation.
That's contrary to sworn statements by Mr Dotcom and his bodyguard contained in the police material.
The Labour Party says Mr Key should make all efforts to discover whether or not he has been misled by Mr Banks, who is a government minister.
"What John Key is doing its putting his head in the sand and pretending that everything's okay," Labour leader David Shearer says. "This will not last. The situation is untenable."
Mr Key is blaming the legislation at the time, which he says was poorly drafted and allowed people to do many things that look odd.
The Prime Minister told Radio New Zealand's Morning Report programme that attacks over this issue are opportunistic politicking by the Labour Party.
'No need', says PM
Later on Monday, Mr Key reiterated his stance, saying he sees no need to seek further assurances from Mr Banks.
Mr Key says he has not read the formal police report on the investigation, or police documents released last week.
He says he will not go back and have a look at the information, because it is not for him to do a forensic analysis of it.
Mr Key says his job is to have or lose confidence in a minister, and to lose confidence he would have to think the minister had misled him, and he has no evidence that Mr Banks has done so.
New Zealand First says Mr Key can only continue to defend Mr Banks for so long, before he damages his own credibility.