National's leader John Key says he believes the Serious Fraud Office will widen its investigation into New Zealand First once it gets started.

SFO director Grant Liddell yesterday announced the SFO would investigate whether donations from Robert Jones and the Vela family reached their intended destination.

He said the investigation would not cover expatriate billionaire Owen Glenn's $100,000 donation to NZ First leader Winston Peters' legal costs, nor claims made under parliamentary privilege that Simunovich Fisheries paid the party money to silence criticism.

But Mr Key today said he believed that once the investigation started it would be widened.

"The director has left himself room to widen out the investigation and I suspect that is what's going to happen," he said today on Radio New Zealand.

He also said that Mr Peters and the party being cleared by the privileges committee and SFO respectively would not be enough for National to work with Mr Peters after the election.

Mr Key said questions still remained around the 2003 parliamentary inquiry into the scampi fisheries quota and whether Mr Peters had misled the public in relation to the Glenn donation.

He also questioned Mr Peters' assertion today that Sir Robert's donation had found its way to NZ First. If that was the case then why was it not declared, he said.

Mr Peters' credibility had been severely dented and he would now find it hard to trust his word.

"We've had so many instances now where Winston Peters' version of events just doesn't stack up with the version of events presented by others.

"Really the call we made on Wednesday to effectively cut Winston Peters and New Zealand First loose wasn't a call we made easily or lightly. We did it with the full knowledge it may well cost National an opportunity to be in government," he said.

"I would find it enormously difficult trusting the word of Winston Peters."