A gunman who fatally shot a senior constable in Queensland's Lockyer Valley had "a grievance against police" and was firing a high-powered gun at specialist officers when he was killed in a shootout, senior police say.

Rick Maddison, 40, was declared dead on Tuesday morning after the gunfight with SERT officers, following an overnight siege in a farmhouse at Adare, near Gatton.

Rick Maddison, 40, shot and killed Senior Constable Brett Forte, police say. ( Supplied )

On Monday, Maddison killed Senior Constable Brett Forte when police tried to stop the fugitive's car on a road at Seventeen Mile in the Lockyer Valley.

Queensland Police Deputy Commissioner Steve Gollschewski said the bomb squad remained at the scene.

He said Maddison had a violent criminal past, but declined to provide further details about the man's history with police.

"This is a person who was highly motivated and had a grievance against police," he said.

"We know that he had issues."

Senior Constable Forte was working with Toowoomba's Tactical Crime Squad when he was shot dead. ( Facebook: Brett Forte )

Deputy Commissioner Gollschewski said specialist officers "followed procedures to the book" while trying to negotiate with Maddison, setting up a cordon around the farmhouse and speaking with him overnight.

"This morning the offender chose to take matters into his own hands. He approached police, he was challenged, he shot at police, he was challenged again, and when pointing his weapon, police returned fire, fatally wounding him," he said.

"Numerous officers called on him to put his weapon down and he chose not to.

"One of our officers received a minor injury, such was the calibre of the weaponry, that it fractured the armoured glass in one of the vehicles and caused a cut to his face."

Commissioner Ian Stewart said an investigation would now take place into the deaths of Senior Constable Forte and Maddison.

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He said he was certain Maddison had been the aggressor and responsible for the officer's death, as well as his own.

"I squarely put everything that's happened in the last 24 hours in his court," he said.

The state's top police officer said it was not clear how Maddison was able to access a high-powered weapon, saying it was "terribly concerning".

"[The] investigation includes components of our homicide squad, our Ethical Standards Command, local detectives, and certainly with oversight by the coroner and ultimately also if necessary, the CCC," he said.

Flowers have been left outside a Toowoomba police station in honour of Senior Constable Forte, a father-of-three who had served with the Queensland Police Service for 15 years.

Forte from family of police officers

Queensland Police Union president Ian Leavers said earlier today Senior Constable Forte had died doing a job he loved.

"That's the only positive we can take out of this — that Brett was doing a job that he loved and he was committed to," he said.

"As we know yesterday — they [the officers] didn't have time to think — it happened that quickly, how it unfolded, and they did what they could.

"Tragically, Brett never had a chance and we've got to live with that and we've got to continue on and support Brett's family and do the best we possibly can there and rebuild."

Flowers are left in memory of a slain police officer at the entrance to Toowoomba police station on May 30, 2017. ( ABC News: Isobel Roe )

Commissioner Stewart said Senior Constable Forte epitomised the police service, and had come from a proud police family.

"His dad was a police officer, his wife is a police officer, his brother-in-law is a police officer," he said.

"His passing yesterday is something that has sent shockwaves across our organisation and will be felt for many days, weeks, months, years to come.

Commissioner Stewart said police would hold discussions with the family and the State Government around funeral arrangements for Senior Constable Forte.

Exclusion zone lifted after 26 hours

On Monday, once Maddison shot at police and killed Senior Constable Forte, he fled into bushland at Adare, near Gatton.

An exclusion zone was declared over several square kilometres, with the standoff between the gunman and police lasting more than 12 hours.

Residents said they were relieved to hear the news the incident was over, with many separated from loved ones since yesterday.

Maddison died after firing at police, Assistant Police Commissioner Mark Kelly said. ( Supplied )

The exclusion zone around the scene was lifted about 5:30pm on Tuesday.

Earlier, resident Terry Hayden, whose wife was among those in lockdown on the other side of the police cordon, said everyone was relieved it was over.

"The community is much safer there now — we didn't realise the extent of how dangerous the man was," he said.