What goes into a terrorism declaration? Questions arise after Perth bomb attack

It took police more than 48 hours to announce it was investigating a man allegedly hurling a homemade bomb into an Invasion Day rally in Perth as a "potential terrorist act".

The 31-year-old was charged with unlawful act or omission with intent to harm (endangering life, health or safety of any person) and making or possession of explosives under suspicious circumstances after throwing the device into a crowd of 2500 people on Forrest Place on Monday.

Indigenous leaders, rally organisers and politicians were among the growing voices who called for the incident to be treated as terrorism before police confirmed the Joint Counter Terrorism Team was investigating the incident last night.

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Independent Senator Lidia Thorpe questioned why police have not shared information about the accused's motivation or ideology.

"On its face, this appears to be a targeted, racist terror attack against First Peoples on our Day of Mourning. That possibility must be taken extremely seriously, not avoided or downplayed," she said.

The Greens' First Nations justice co-spokesperson Jess Beckerling criticised the "soft language and low-level charge" by police.

WA Police said they could not comment as the matter was before the court.

But Australian Strategic Policy Institute national security program director John Coyne explained that police needed to prove "beyond a reasonable doubt" that the motivation was to commit terror and for the device to detonate in order to make a declaration.

"Some people will say it's self-evident," he said.

"The police and the prosecutors will need to be able to go into a courtroom and prove beyond reasonable doubt that their motivation was to create terror and fear based on an ideology.

"That takes a while to gather that sort of information."

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Coyne said there was an air of caution to declare an incident terrorism and pointed to the bombs found inside a caravan in Dural, Sydney, in January last year.

The incident was declared terrorism, but police later found the caravan never intended to detonate and was created to sow fear.

"We want to make sure that the decision is made with as much information as possible. We don't want to potentially derail future prosecutions. We don't want to have undue worry in the community," Coyne said.

Police alleged the "rudimentary" bomb thrown in Perth contained ball bearings and screws, which were wrapped around a liquid in a glass container.

Officers arrested the man, who allegedly indicated the device may contain explosives.

It failed to detonate, but the crowd was evacuated. No one was injured.

The 31-year-old has been remanded in custody and will face the Perth Magistrates Court on February 17.

The accused's identity has been suppressed by the court due to concerns for his safety.

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