The trio's bodies were discovered in their home at Biddeston, about 25 kilometres west of Toowoomba, by a family member just before 7pm on Monday.

The woman, 27, and her father, 52, had been shot in the head at close range.

The boy's body was found in his bed, with no obvious cause of death.

His grandfather's body and a gun believed to be the murder weapon were found close by.

Police are currently treating the deaths as murder-suicide but say investigations are continuing to determine if a fourth party was involved.

Detective Inspector Dave Ishwerwood said the the body of the pregnant woman was found in the bathroom, while the bodies of the boy and his grandfather were both found in a bedroom.

"Two of the victims, both the adult male and female, had very serious wounds to their bodies. Both appear to have been shot in the head," he said.

But he said the boy's cause of death was still yet to be determined.

Inspector Isherwood said neighbours reported hearing three distinct shots about 10.30pm Sunday, which he said were believed to have come from a high calibre gun.

"There is nothing to suggest there was any incident that occurred in terms of a disturbance, very simply, two people who live in the town heard three distinct gunshots," he said.

"There's no evidence the house has been broken into. When police arrived they had to force entry, so it appears the house was secure."

Earlier, Inspector Isherwood said police were treating the deaths of the woman and her son as suspicious but the death of the man as non-suspicious.

He said despite the discovery of the murder weapon in the house, investigations were continuing into who fired the fatal shots.

"They are high calibre weapons, as to whether it was aimed by one of the residents of the house or a resident attached to the house, we don't know," he said.

Forensic experts will continue to probe the crime scene today, he said.

EARLIER A woman found her seven-year-old nephew, heavily pregnant sister and father shot dead in a home near Toowoomba on Monday night, according to a neighbour.

"She was the one who found them and she came out screaming and shouting, 'they have all been murdered,'"  said Paul Farmer, the owner of the Biddeston Store, which is located across the road from the house where the bodies were found.

The trio was found dead of apparent gunshot wounds at their home in the small farming community, 25 kilometres west of Toowoomba, just before 7pm on Monday.

Detective Inspector Dave Isherwood said police are treating the deaths as an apparent murder-suicide.

"I must stress that we are in the early stages of our investigation however it would appear that two of the deaths are suspicious and the death of the man is non-suspicious," he said.

The small farming community of Biddeston, which has a population of just more than 300, was in shock on Tuesday morning, following the discovery of the bodies, Mr Farmer said.

Mr Farmer said the woman's hysterical screams on Monday night prompted many neighbours to run from their houses to her aid.

He said the man found dead, aged 52, lived in the home with a number of his children, following the death of his wife about four years ago.

The man's 27-year-old daughter and seven-year-old grandson were both also killed.

The woman, who police said was "visibly pregnant" was the boy's mother.

Mr Farmer said the father was well liked in the town and he and many others had trouble believing the man could be responsible for the deaths of his daughter and grandson.

"He was that sort of guy who would do anything for anyone, I have never seen him angry or lose his temper," he said.

Mr Farmer said he saw the man almost daily in his store with his grandson.

"I see (the man) every day, he would come in hand-in-hand with (the little boy) and buy him lollies," he said.

"The way (he) was with (the boy), he worshipped him, he was a cute little kid."

The boy attended the local school, Mr Farmer said, which has a student population of just 57.

"The school will be sad today because Jack hasn't been there, he's usually outside there about now waiting for the bus," he said.

Mr Farmer said the 27-year-old woman also found dead lived at the house.

He said police were regularly called to the house to attend domestic incidents but they stemmed largely from the dead man's sons.

Police were called to the Biddeston Southbrook Road home shortly after the discovery of the bodies, where a crime scene was quickly established.

Detectives and forensic investigators remained on scene throughout the night, combing for clues.

A police cordon remains in place.

Investigations are expected to continue throughout the day and an Investigation Centre has been established at Toowoomba Police Station.

Mr Farmer said the man believed to be responsible for the killings regularly mowed his lawns for him.

He said he last saw him on Sunday, when he gave no inkling of any trouble.

"He was quite house proud really, he came and did my lawns and I used to give a pouch of tobacco," he said.

"Only on Sunday he was saying to me, 'my mower's broken but as soon as I get it fixed I'll be over'.

"We always thought nobody knows where Biddeston is and this has put it on the map for all the wrong reasons."

Correction: An earlier version of this story incorrectly reported the boy's mother found the bodies. She was among the victims. 

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