Captive ... Simon Peterffy, Geoff Tuxworth, and Glen Pendlebury.

Simon Peterffy, left, with fellow Forest Rescue activists Geoff Tuxworth and Glen Pendlebury. Photo: Sea Shepherd Conservation Society

A high-profile anti-whaling and environmental activist has been given a suspended jail term for bursting into the offices of Western Australia's Forest Products Commission and throwing rancid butter.

Perth magistrate Steven Malley told Simon Peterffy, 44, he was being given "the keys to unlock yourself", after suspending his six-month-and-one-day jail term for 12 months on Monday.

"I don't doubt your cause, but I do have to doubt your methodology," Mr Malley told Peterffy.

Peterffy had pleaded guilty to one count of criminal damage and one count of trespassing.

For the trespass, he was fined $1500 and ordered to pay $64 costs and $4793.25 restitution to the FPC for having to clean its offices.

Another man who appeared with Peterffy, Rowan Davidson, 57, was fined $750 plus costs after pleading guilty to trespass.

Two other men, Peter Roost and John Vukovich, had previously been fined $750 each for their part in the same incident, while another man, Rodney Ashmond, is yet to go to trial.

All the men except Ashmond are members of the environmental activist group Forest Rescue, and had gone to the FPC's head office in Rivervale, in Perth's southern suburbs, on May 20 last year to confront acting general manager David Hartley.

The group was carrying two bags of rancid butter and a copy of the FPC's annual report, which was soaked in the putrid substance and thrown on Mr Hartley's desk as the men shouted "Your report stinks!"

The phrase was in reference to the FPC's loss-making logging of native forests in WA's southwest region.

In sentencing, Mr Malley said Peterffy had been a "principal player" in what the convicted men had described as a "prank gone wrong".

He said a deterrent sentence was needed and warned Peterffy he would go straight to jail if he committed a similar offence.

After sentencing, Peterffy said the group had wanted to highlight logging of native southwest forests and its threat to the critically endangered numbat.

"We're not going to be deterred from trying to save a species from extinction," he said, adding he would continue to protest.

Peterffy was one of three Sea Shepherd crew who boarded a Japanese whaling vessel off WA's southwest coast in January this year in an attempt to delay it.

No charges were laid over that incident after the three were transferred to an Australian Customs Vessel and dropped off in the southern port city of Albany.

AAP