Six people, including two young police constables, have been shot dead in an "absolutely devastating" ambush and siege at a remote Queensland property.
Constables Matthew Arnold, 26, and Rachel McCrow, 29, died along with 58-year-old neighbour Alan Dare after being shot at Wieambilla in the Western Downs region on Monday.
Three alleged killers - a former NSW school teacher, his brother and sister-in-law - died in a firefight with heavily armed tactical officers later in the evening.
Queensland Police Commissioner Katarina Carrolll says it is an extremely emotional and challenging time for the entire police service.
"Losing one of our own has a profound impact on every single officer and their families,'' she told reporters on Tuesday.
"To lose two officers in one incident is absolutely devastating.
"This event is the largest loss of police life we have suffered in a single incident in many years."
Prime Minister Anthony Albanese paid tribute to the slain constables, Mr Dare and the entire "close-knit and caring" Queensland community, saying it was a tough day for all Australians.
"This is, indeed, a devastating day for everyone who loved these Australians," he said.
"This is not a price that anyone who puts on the uniform should ever pay. We can never count the true cost.
"My heart goes out to the families and loved ones of all those affected by this tragedy. With honour they served, and Australia mourns with you today."
Four police officers went to the Wieambilla property to look for missing NSW man Gareth Train at 4.30pm on Tuesday after being asked by police in that state.
The 46-year-old, his brother Nathaniel Train, 47, and his 45-year-old sister-in-law Stacey Train opened fire on the officers after they jumped a fence.
A "hail of bullets" struck constables Arnold and McCrow, who fell to the ground, with 28-year-old constables Keeley Brough and Randall Kirk taking cover before escaping.
The three Trains then walked up to constables Arnold and McCrow and shot them both at point-blank range while they lay on the ground.
"The ruthless, murderous trio have ... executed the two police, our fallen officers, who were on the ground. They have executed them in cold blood," police union boss Ian Leavers told ABC Radio National on Tuesday.
An emotional Ms Carroll says her officers "never stood a chance" in the ambush.
"The fact that two got out alive is a miracle," she said.
"To think they survived the scene, let alone ... called for assistance, is just extraordinary."
The shooters then lit a fire to burn or smoke Constable Brough out of her hiding place in the long grass and into the open.
Mr Leavers said the officer, who had only eight or nine weeks of experience, believed she was going to be shot dead or burnt alive.
"I do know she was sending messages to loved ones saying she almost was at a point where she thought it was her time," he said.
Neighbour Alan Dare, who had come out of his property to check on the grassfire, was also shot in the back by the trio "in cold blood as well" and died.
Sixteen police officers then tried to rescue their colleagues, but were met with heavy gunfire and had to retreat.
Specialist police were called in, as were police helicopters, which the trio also fired at, before they were killed in a gunfight about 10.30pm, local time.
Social media posts have linked Gareth Train to an online conspiracy community, and sources have told AAP that Nathaniel had been living "off the grid" for more than 12 months.
Flags on Queensland government buildings will fly at half-mast, and the Story and Victoria bridges in Brisbane will be lit blue and white in memory of constables Arnold and McCrow.
"I doubt that this will bring much comfort to the families and loved ones of those who were killed," Queensland Premier Annastacia Palaszczuk said.
"They were so young, they were incredibly brave. These were absolutely callous, brutal acts."
