Six people – including two young police constables – have been shot dead in an ambush and siege at a remote Queensland property that has left the tight-knit local community reeling.

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 Darling Downs region on Monday evening.

Three killers – a former school principal, his brother and sister-in-law – died in a firefight with heavily armed tactical officers later that night.

Queensland Police Commissioner Katarina Carroll said the officers’ deaths meant it would be 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 not a price that anyone who puts on the uniform should ever pay. We can never count the true cost,” he said.

“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 Queensland officers went to the Wieambilla property to look for missing NSW man Nathaniel Train at 4.30pm on Tuesday after a request from interstate police.

The 46-year-old, his brother Gareth Train, 47, and Gareth’s wife Stacey Train, 45, opened fire on the officers after they jumped a fence.

A “hail of bullets” struck constables Arnold and McCrow, who fell to the ground, while 28-year-old constables Keeley Brough and Randall Kirk took cover before escaping.

Constables Arnold and McCrow were both shot dead 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,” police union boss Ian Leavers told ABC Radio.

“They have executed them in cold blood.”

An emotional Ms Carroll said the 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 lit a fire to burn or smoke Const Brough out of her hiding place in the long grass surrounding the property.

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”.

Sixteen police officers tried to rescue their colleagues, but were met with heavy gunfire and had to retreat.

Specialist police were called in, as were police helicopters, before the trio were killed in a gunfight about 10.30pm, local time.

Social media and other online posts link Gareth Train to a conspiracy theory community, while sources have told AAP that Nathaniel Train, who worked as a principal at schools in Queensland and NSW, 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.

Queensland Premier Annastacia Palaszczuk said she doubted the tributes would bring much comfort to the families and loved ones of those who were killed.

“They were so young, they were incredibly brave. These were absolutely callous, brutal acts,” she said.

An emergency declaration remains in place for the Wieambilla area and a crime scene has been established at the property.

Prime Minister Anthony Albanese earlier said it was a heartbreaking loss for the families and friends of those who had lost their lives in the line of duty.

“My condolences to all who are grieving tonight – Australia mourns with you,” he tweeted.

Opposition leader Peter Dutton, a former Queensland police officer, was also moved by the tragedy.

“Deeply distressing news coming out of western Queensland tonight with those police officers who have been murdered,” he wrote.

Premier Annastacia Palaszczuk said the deaths were a “horrible tragedy” as Western Downs regional council mayor Paul McVeigh said his community was shattered by what was “an absolute tragedy in our community”.

By Nick Gibbs and Robyn Wuth in Brisbane