A shooting ambush in a remote Australian town that left six people dead has been declared a religiously-motivated terrorist attack.

Authorities say it is the first time Christian extremist ideology has been linked to a terror attack in Australia.

Two police officers and a neighbour were killed when Nathaniel, Stacey and Gareth Train opened fire on a rural property in Queensland last December.

The trio were shot dead after a lengthy stand-off with police.

Police have been investigating whether the group - brothers Nathaniel and Gareth, and Stacey, who had at different times been married to each man - were linked to conspiracy theories.

Queensland Police Deputy Commissioner Tracey Lindford on Thursday said their investigation had found the Trains "acted as an autonomous cell" and "executed a religiously-motivated terrorist attack".

They subscribed to a Christian fundamentalist belief system known as "pre-millennialism", and had targeted police.

"They did refer to police as monsters and demons - as evil."

"Christian extremist ideology has been linked to other attacks around the world, but this is the first time we've seen it appear in Australia," Ms Lindford said.

The attack was premeditated, she said, and investigators had found "significant evidence" of advance preparation and planning.

The property - owned by Gareth and Stacey - had been set up with camouflaged hideouts, barriers, dirt mounts, guns, knives, CCTV and mirrors on trees.

While there's "no evidence" that any one else in Australia participated or assisted in the attack, Ms Linford said the Trains have been linked to individuals in the United States. Police have shared information with investigators there.

"They'll determine what investigations they might make as a result of that information," she said.

Police had travelled to the remote inland property - about 270km (168 miles) west of Brisbane - on 12 December to check on Nathaniel Train who had been reported missing in New South Wales.

Four officers were inundated with gunfire after leaving their cars and approaching the house, owned by Gareth and Stacey.

Two constables - Matthew Arnold, 26, and Rachel McCrow, 29 - were hit immediately, then reportedly shot again, execution style. Another officer was injured but escaped, while the fourth was terrorised by the shooters who lit fires to try and flush her out.

A neighbour, 58-year-old Alan Dare, who turned up at the property to help was fatally shot too.

Ms Lindford said there was "not one catalyst" for the trio's extremism.

But Nathaniel's 2021 heart attack "was a profound moment for him and his belief in God", she said. Gareth and Stacey losing their school jobs due to Covid-19 vaccine mandates also increased their anti-government views.

Ms Lindford said their mental health had also been considered by investigators, but ruled an unlikely factor.

"We quite often do see in our terrorist investigations, people who are impacted by mental health because they are easily radicalised."

"When you've got three acting together, it's challenging to say that there's a mental health issue in this instance."

The attack will be investigated during a coronial inquiry, which will make final determinations on the Trains' motive, the police commissioner added.