The Arson Squad is investigating after fire damaged heavy machinery at a controversial housing development site in Golden Bay.
Vandals threw a molotov cocktail type device over a fence at around 9:45 last night, causing $400,000 damage to earth moving equipment.
Firefighters contained the blaze within 30 minutes.
Police searched the area however no persons were located.
Demonstrators have been at the site for several days protesting the bulldozing of sand dunes to make way for a housing development.
The Western Australian Planning Commission granted conditional approval for the subdivision of the area, known as 5A, in June this year, with plans for 88 residential lots.
Mandurah detectives are seeking anyone with information about the incident to contact Crime Stoppers on 1800 333 000 or report on line to www.crimestopperswa.com.au
PIC: Supplied
Mandurah Crab Fest boasts huge range of free, family friendly entertainment
Fuel prices surge in Perth, Mandurah
Teen boys charged over Mandurah burglaries
Tragic update in search for missing man Robert Snyder
Premier Roger Cook to leave Kwinana electorate, move to Perth
Mandurah Marine Rescue's new vessel named in memory of nun who survived 1959 boating accident
Teen dies in motorbike crash at Waroona Dam
Shark alert issued after whale carcass reported at Preston Beach
Kwinana businesses to refuse serving students during school hours