Police say two of their officers were executed “in cold blood” as they lay on the ground by a “ruthless murderous trio” who sparked an hours-long siege on a remote Queensland property.

Six people are dead, including the two police and a neighbour, after the brutal ambush and deadly shootout came to a dramatic end late on Monday.

The “terrible scenes” unfolded in Wieambilla, 300 kilometres west of Brisbane, after four police were sent to investigate reports of a missing NSW person on Monday afternoon.

Queensland Police Union president Ian Leavers said the officers “didn’t stand a chance” as they arrived and were “inundated with gunfire”.

Two of the young officers in their 20s fell and were “executed in cold blood” by the perpetrators as they lay on the ground, Mr Leavers told ABC on Tuesday morning.

“In the modern era of policing, we haven’t seen police executed like this,” he said.

The other two police managed to escape and call for help, with a female officer who had been in the force only for about eight weeks hiding in the grass.

A neighbour was also “shot in the back” and killed, Mr Leavers said, amid reports the man walked up to a gate to investigate what appeared to be a car fire next door.

The gunning down of officers, which happened about 4.45pm, triggered a massive police response as a special operations team descended on the isolated property on Wains Road.

It ended with a dramatic shootout about 10.30pm on Monday when three people – two men and a woman – were “fatally shot”, police said in a statement early on Tuesday.

Mr Leavers said 16 front-line police had gone in to “retrieve their colleagues” with the hope of saving their lives, but it was too late.

He said the deceased officers “had no idea what they were walking into, what would be the end of their lives” when they responded to a call about a missing person.

“It was just completely unexpected, and they didn’t have an opportunity.”

An emergency declaration remained in place on Tuesday morning for the Wieambilla area on the Western Downs. A crime scene has been established at the property.

The two dead police officers were reportedly a 29-year-old man and 26-year-old woman.

The other male officer suffered a gunshot wound to the leg and the female officer hid on the property after managing to escape.

Before the drama had ended, Police Commissioner Katarina Carroll told media of the police deaths and said it was the “largest loss of life we have suffered in one single incident in recent times”.

Fighting back tears late on Monday, she said the horrific incident was devastating for the victims’ families and the entire organisation.

“It is sadly a reminder of the unpredictable nature of policing and the incredible dangers our officers face while protecting our community.”

Circumstances surrounding the incident will be investigated by police and overviewed by the Ethical Standards Command on behalf of the coroner.

Prime Minister Anthony Albanese said it was a heartbreaking day for families of the fallen officers after the “terrible scenes in Wieambilla”.

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

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.”

Tweet from @AlboMP

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

“This is a massive trauma in our region,” Cr McVeigh said.

-with AAP