Six dead, including two officers, after ambush on Queensland police at Wieambilla

Their alleged killers - a former school teacher, his brother and sister-in-law - were later shot dead in a firefight with heavily armed tactical officers.

The trio were killed in the terrifying final moments of a deadly siege on Queensland's western Darling Downs on Monday night, which earlier claimed the lives of the two junior police and an innocent bystander.

Constables Matthew Arnold, 26, and Rachel McCrow, 29, were investigating reports of a missing person at 4.30pm when confronted by a "hail of gunshots" after jumping a fence.

The pair were shot and fell to the ground while two of their colleagues took cover behind them.

The two other officers who responded to the missing person's call, Constables Keeley Brough and Randall Kirk, both 28, were taken to hospital with non-life-threatening injuries.

Neighbour Alan Dare was also shot and killed by the armed offenders.

An emotional police commissioner Katarina Carroll said the officers "never stood a chance".

"The fact that two got out alive is a miracle," the commissioner said on Tuesday.

"To think they survived the scene, let alone ... called for assistance, is just extraordinary."

Police union boss Ian Leavers said the officers were executed.

"The ruthless, murderous trio have ... executed the two police, our fallen officers, who were on the ground. They have executed them in cold blood," he told ABC Radio National on Tuesday.

The three people who confronted police at the property at Wains Road, Wieambilla, then allegedly lit a fire in an effort to force the young policewomen hiding in long grass into the open, he said.

"She actually believed that she was either going to be shot or she was going to be burnt alive.

"A member of the public, a neighbour, he went to investigate the burning grass and he was shot in the back in cold blood as well."

A siege situation ensued, with specialist police and PolAir responding, before coming to a bloody final end hours later.

"Two males and a female were fatally shot during a confrontation with police shortly after 10.30pm," the force said in a statement early Tuesday morning.

The alleged killers have been identified by senior police sources as Nathaniel Train, his brother Gareth and wife Stacey.

All three are considered to have been offenders actively involved in the ambush that unfolded as the junior officers arrived at the property.

The quartet had been sent to the remote location following a request from NSW police trying to locate Nathaniel Train, missing from his western NSW home for almost a year.

"That request was given to the local police to go to this address to check on that missing person," Ms Carroll said.

People had been in touch with the missing man before contact was lost in recent days.

READ MORE:

Ms Carroll said the incident was devastating for the victims' families and the entire organisation.

She fought back tears as she confirmed the initial deaths of the three people, including her officers, at a media conference late Monday night.

"I would like to also pay tribute to all police and first responders who assisted at the scene and are continuing to respond this evening," she said.

"Tragically, this is the largest loss of life we have suffered in one single incident in recent times ... It is devastating news."

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.

"Police officers face danger every day to keep us from it."

Premier Annastacia Palaszczuk said the deaths were a "horrible tragedy" as Western Downs regional council mayor Paul McVeigh said his community was shattered.

"It's an absolute tragedy in our community," Cr McVeigh said.

Australian Associated Press