Let's face it, there is never a good time to be referred to the Serious Fraud Office.

And there may certainly have been a worse time for the National Party to be referred to the SFO - a month before a general election, for example.

But the decision by police to refer a complaint about donations made by ex-National MP Jami-Lee Ross to the SFO will be damaging - no matter what the outcome and especially now.

It comes at a time when leader Simon Bridges was starting to gain traction against Jacinda Ardern over the proposed capital gains tax.

And it comes at a time when Ardern has been forced to defend some pretty D-grade behaviour by New Zealand First minister Shane Jones.

Ironically, this week was probably the strongest Bridges has looked in his job since his parliamentary travel expenses were leaked on August 13 last year - most likely by Jami-Lee Ross if the evidence is correct.

Following the setbacks of last year, expelling Ross from the caucus for gross disloyalty, and the setbacks from polling results in January and February, Bridges looked to be in recovery mode.

It now appears to be a case of one step forward, three steps back. Despite his composure moments after reading the police press statement, it will have hit Bridges like a 10-tonne truck.

That is not to suggest that Simon Bridges has done anything criminal, or even wrong, but there is no avoiding the stigma of having the SFO looking into your party's affairs.

Just ask Winston Peters, who was subject to an SFO inquiry in 2008 over electoral donations which, together with an unrelated privileges committee hearing into his non-disclosure of a $100,000 donation from Sir Owen Glenn, saw Peters and New Zealand First tipped out of Parliament at the 2008 election.

It may be months before the result of SFO inquiries are known and the stigma will give Bridges no clear air.

Given the relationship between the police and Jami-Lee Ross, it may have been advisable for the police to have transferred the case to the SFO earlier.

The SFO's mandate extends to cases deemed to be in the public interest and this certainly fits the bill.

The police are the only organisation who can confirm 100 per cent that the phone used to leak Bridges' travel expenses last year belonged to Ross.

They were able to track down the culprit and assure Bridges that the leaker had necessary support for mental health issues.

Bridges' inquiry into the leak showed that, on the same day police gave him those assurances about the health of the leaker, Jami-Lee Ross had been in repeated phone contact with the police in his area.

The National Party caucus will reserve judgment on Bridges until the case is resolved one way or the other. Whether the public will do the same for National is more unlikely.