Winston Peters is accusing the Serious Fraud Office (SFO) of unfairly scrutinising the New Zealand First Foundation over other parties under investigation.
The New Zealand First leader is taking the SFO to court for laying charges against two people from the foundation so close to the election.
Mr Peters told The AM Show on Wednesday no other party has been treated this way.
"Never before have they put a timeline on an investigation. Why not Labour? ... They walked in, grabbed people at their work, grabbed their phones, their laptops and what have you, without them having a chance in many cases to talk to their lawyer."
Peters said the SFO was going to announce the charges a week ago, but was forced to reveal no NZ First MPs, party members, staff or candidates were involved after he laid an injunction.
His request for the charges to be kept secret until after the election was rejected.
"They cannot explain their behaviour... Why is the Labour Party, or other parties, exempt from this behaviour? And NZ First the only one that you're determined to have this matter resolved on?"
Peters says the foundation was modelled on the National Party's similar fundraising vehicle.
"I've got no connection with it whatsoever, I've been flat out. I don't know what the daily operations are - but I just know they were told to keep it legal. I'm not there to defend them... If ours is the same structure as theirs, why is the National Party not being investigated?"
The National Party has been investigated, and four people have been charged - including current Advance NZ leader Jami-Lee Ross, who used to collect donations for National. The SFO is also investigating donations made to Labour in 2017, but has laid no charges yet.
It's also still investigating a donation made to Auckland Mayor Phil Goff in 2016 and donations to Christchurch Mayor Lianne Dalziel in 2019.
National leader Judith Collins said Peters' actions made him look "very desperate".
"The thing is, if they didn't put it out before the election they'd be accused of trying to play politics with it - and then they put it out before the election, so they get accused of trying to play politics with it. Whatever they do was going to be wrong and he was going to fight it."
She said the SFO had the right to go about their business without interference from MPs.
"I thought it looked like the end actually, for his political career. I think it is. Seriously, I've been involved in politics now for almost 20 years - I looked at him and I thought, I've never seen him look that bad."
Collins said his polling - at about 2 percent was a "tragedy for him personally [but] probably a good thing for the country".
