A law professor says that there is currently no smoking gun to implicate Simon Bridges in fraud allegations.
Police today said they had referred the handling of a political donation to the SFO, based on a complaint made by former National MP Jami-Lee Ross.
Ross, who last year spectacularly quit the party after falling out with leader Simon Bridges, told media he was happy to co-operate with the agency.
Bridges earlier said he is confident his hands are clean and it is the party that has questions to answer.
And he added that he had no concerns about the party as he believed it had good processes to deal with political donations.
Law professor Andrew Geddis says that while it is not a good look for anyone to be investigated by the Serious Fraud Office, he told Larry Williams that Bridges may not even be properly investigated.
He says it raises the question about what the police investigation has turned up.
"It may be that they think someone from within the National Party, whether it's Simon Bridges or someone else, has done something to hide this donations. It may the actual donations themselves may have come from places other then they appear to on the surface."
Geddis says that under the Electoral Act, the general secretary is personally held liable for illegal or corrupt practices involving party donations returns.
Earlier today, Jami-Lee Ross addresses media, saying that he is pleased that his complaint is being looked into.
"Every time I've been told I'm wrong or baseless, I come up with something," Ross told reporters this afternoon.
"Simon was offered the $100,000 donation on the 21st of May. I was not there on the 21st of May."
He said it was understood the police investigation got to the point where it was appropriate for the SFO to become involved.
Asked whether he had any proof of his claims, Ross did not directly answer the question.
Ross said no one suggested the donor split the donation but was sufficiently aware of the rules.
But he said the onus was on the party, not the donor, to ensure the rules were followed.
Ross said it was several smaller lots of money that added up to $100,000.
Geddis says that Ross has distanced himself from the complaint and has been less than forthcoming about his own role.
"He's claiming that Simon Bridges made it clear to him that he wanted the money broken up in a certain way.W hat he hasn't talked about publicly is whether he went back to the donors and gave them any instructions, 'this is how we want it done'."
He says it's hard to see how Ross would not have been involved.
Ross, who quit and was kicked out of the National caucus last year, lodged a complaint with police about what he alleged was a $100,000 donation to the National Party from businessman Yikun Zhang that was then split into smaller amounts to hide it.
Bridges has strongly denied that, as has the National Party hierarchy which has said the $100,000 sum cited by Ross was made up of eight separate donations from different donors.
There is no suggestion of any wrongdoing by Zhang.
Ross alleged Bridges, MP Todd McClay and party general manager Greg Hamilton all knew about the donation being split up.
"Simon asked me to collect this donation. He was at pains to point out the donation should not be made public and could I ensure this," Ross said last year of the donation.
Ross said he did as Bridges asked, splitting the money into chunks smaller than the $15,000 limit at which donations had to be declared.
But Bridges told reporters this morning he had not instructed anyone to break up the $100,000 donation, nor has he said anything that could have been interpreted that way.
He said the matter seemed to have nothing to do with him, and he was completely confident his hands were clean.
"I've seen the statement (from police). On the face of it, it seems to be about the National Party. The SFO is investigating. I think there's questions for them to answer and you'd hope in due course they'll do that.
"I expect the National Party to fully cooperate."
He was not going to seek clarification from the SFO that he was not involved.
"I would have expected, if that was the case, that they would have been in contact. They haven't."
He did not have concerns about the National Party as he believed the donation had been handled lawfully.
A spokesman for the National Party said neither president Peter Goodfellow nor Hamilton had any comment at this stage.