Former National MP Jami-Lee Ross told the police he sounded the alarm over a donation to the party because he thought it was dodgy and potentially illegal.
The High Court in Auckland has heard a 2018 police interview in which Ross claimed then party leader Simon Bridges knew of the alleged deceptive donation.
It's the third week of a trial over anonymous donations to the country's two biggest political parties in which seven people, including Ross, are defending charges relating to electoral fraud.
For the National Party case, the Crown is alleging that two $100,000 donations, one in 2017 the other in 2018, were split into smaller amounts among "sham donors" to avoid triggering the disclosure limit, and hide the true donor.
When he laid a complaint in 2018, Ross handed over the names of the people he claimed split one of those donations.
Ross told the police why he recorded a phone call with Simon Bridges, in which he acknowledges a $100,000 donation.
"I recorded it for that reason because I could smell danger, political danger, but legal danger too," Ross said.
"My thoughts were this is a bit dodgy, potentially in breach of the law. I felt uncomfortable by it... I need to have some evidence if this comes back to bite the party in the backside."
Ross claimed Bridges had asked him to collect the donation, which arrived in the Botany electorate account as one lump sum.
"There's a direct link to me and if shit hit the fan, to use that expression, it would all have landed on me," he said.
"I wanted to ensure I had evidence that the party leader asked me to do this."
But when Ross laid the complaint with the police, he set off a chain events resulting in him being charged by the Serious Fraud Office.
On Monday, the court heard how Ross's mental health spiralled and he was hospitalised within days of laying the police complaint.
In his opening statements, Crown lawyer John Dixon QC said Ross was the party insider who helped facilitate the split donations.
"With the help of a party insider the sham donors were represented to each party to in fact be the true donors," Dixon said.
"The result of this scheme was the true donors made a large donation to the party which was known by one or more key insiders within that party... but the party secretary the Electoral Commission the public at large were kept in the dark."
Bridges, a former Crown prosecutor, has denied any wrongdoing in relation to donations to the party.
He will be called as a witness in the trial.