A 20-year-old man has been charged with five offences, including preparing for a terrorist act, after his online activity was investigated by police.
Police said they had carried out a search at a home in Bindoon, about 80km from Perth, where officers allegedly seized firearms, imitation firearms, a knife collection, ballistic vest, gas masks, and lock-picking equipment.
WA Police Commissioner Col Blanch told reporters that officers allegedly also found a notebook written as a 'manifesto-style document'.
It 'outlined preparations for a terrorist attack at significant locations, including the WA Police Headquarters, Parliament House and places of Muslim faith', he said.
The raid on the Bindoon home followed a probe by the WA Joint Counter Terrorism Team into the young man's online activity, officers said on Friday.
Commissioner Blanch said police 'received intelligence about some communications on an encrypted communications platform' in January and February.
He alleged the messages contained 'white supremacy ideology' and 'anti-Muslim ideology', 'antisemitism' and other conversations about minority groups, which he described as 'abhorrent'.
Following the raid on the Wheatbelt property, the 20-year-old from Bindoon, who lived with his parents, was charged with acting in preparation for a terrorist act.
A 20-year-old man in WA has been charged with five offences, including preparing for a terrorist act, after his online activity was investigated by police
Police allegedly seized weapons while raiding the man's Bindoon home
Premier Roger Cook said on Friday that the young man's alleged plan had been for a 'mass casualty event' in Perth, including at mosques, Parliament House and police headquarters
He was also charged with one count of using a carriage service to menace or harass and one count of possessing a prohibited weapon. He is also accused of two firearms offences.
Premier Roger Cook said on Friday that the young man's alleged plan had been for a 'mass casualty event'.
He said the counter terrorism team 'had prevented an alleged act of terror in our city' to 'attack the heart of our democracy'.
'Whatever our differences, we all have the right to go about our lives free from the fear of violence,' he said.
After an appearance at Perth Magistrate's Court on Friday, the 20-year-old's bail was refused while the matter remains before the court.
Commissioner Blanch said police believe the plot was a contained event and do not believe the wider community is at risk.
Australian Federal Police Superintendent Peter Hatch added that it was the first time a person had been charged with preparing for a terrorist act in WA.
The arrest comes just a month after 31-year-old Liam Alexander Hall was accused of throwing a homemade bomb into an Invasion Day rally crowd.
The arrest comes just a month after 31-year-old Liam Alexander Hall was accused of throwing a homemade bomb into an Invasion Day rally crowd
The incident has been declared an act of terror - the first in Western Australia's history
The incident has been declared an act of terror - the first in Western Australia's history.
Thousands of people were evacuated from Forrest Place in Perth's city centre after police found an object containing volatile chemicals, nails and metal ball bearings.
Magistrate Lynette Dias later lifted a suppression order on Hall's identity on February 17 after hearing submissions from a commonwealth prosecutor and lawyers for various media outlets.
The matter was adjourned to March 31 after a Commonwealth charge of engaging in a terrorist act was added to two state charges.
If Hall is found guilty of the terrorism offence, he faces life behind bars.
He is yet to enter pleas to any of the charges, which also include an unlawful act or omission with intent to harm and making or possessing explosives under suspicious circumstances.
