A Queensland grandfather shot himself after killing his young grandson and pregnant daughter at their home near Toowoomba earlier this week, police believe.

The bodies of Derek Sharpley, 52, his daughter Kris-Deann Sharpley, 27, and her son Jackson, 7, were found on Monday by a relative at their Biddeston home, 25 kilometres west of Toowoomba.

Ms Sharpley was heavily pregnant and was found in the bathroom of the house with a gunshot wound to her head.

There was no sign of a struggle and police said it was likely Derek had killed her with a high velocity .223 rifle at close range before shooting himself in the head some time later.

"It is reasonable to expect that as [Derek had] a self-inflicted wound that he could be the perpetrator for the deaths of the other two people," Detective Inspector Dave Isherwood said.

"It is highly likely."

Mr Sharpley's body was found in a bedroom alongside Jackson's body.

The child was in bed and did not have any noticeable physical injuries.

"Suffocation is often very hard to prove, but if you think if it was strangulation there would be some evidence to that effect you would think," Detective Inspector Isherwood said.

"Until the pathologist gives us a report on what he thinks the determination is, we're up in the air as to cause of death.

"We cannot rule any cause of death out."

The post-mortem on Jackson was inconclusive and it could take some time to determine his cause of death.

"It could take up to six months," Detective Inspector Isherwood said.

"If anybody thinks it is going to be in a couple of weeks they are going to be disappointed.

"Quite often some of the tests can't be done locally."