The fishing and bloodstock millionaire Philip Vela said the SFO investigation would be "like trying to pick fly shit out of black pepper".
Asked what he meant, the Switzerland-based Mr Vela asked the Herald "have you tried it?"
He then said "that's not a comment" and "regrettably" declined to comment further.
He would not say if he had been contacted by or was co-operating with the SFO.
Mr Vela and his brother Peter have reportedly made donations of more than $150,000 from six different accounts over four years (1999-2003) to the New Zealand First party.
The SFO investigation is looking at whether the donations "did not reach their intended destination".
Sir Robert Jones said the investigation was "appropriate and timely".
Sir Robert donated $25,000 believing it was going to New Zealand First but found it had gone to the Spencer Trust.
But Sir Robert said he believed this was "innocuous" compared to the other allegations.
Sir Robert said he was not overly concerned about finding out where the money actually went. "You buy a bloody poppy on poppy [Anzac] day and some ratbag puts it in his pocket ... You wouldn't feel angry about your dollar, you'd feel angry about him."
NZ FIRST'S WEALTHY DONORS
* The publicity-shy Vela brothers have an estimated wealth of $180 million.
* The origins of their wealth is a fishing business begun by their Croatian father. Vela Fishing is now one of New Zealand's largest privately owned companies.
* The brothers have also turned an interest in horse breeding into an empire, owning Pencarrow Stud near Cambridge and NZ Bloodstock, the company that runs the Karaka yearling sales.
* They spend most of their time in Geneva but own a seaside home at Opito Bay on the Coromandel.
SIR ROBERT JONES
* Sir Robert's property empire is estimated at $250 million. His portfolio includes buildings in downtown Auckland, Wellington and Sydney.