A police officer who has been jailed for killing his baby son with a single punch is still a serving member of the Queensland Police Service (QPS).

Colin David Randall was suspended when he was charged two years ago but cannot be discharged or dismissed from the QPS until a formal process takes place.

The Queensland Police Union (QPU) said it regarded Randall's offence as the "most serious of crimes" and it would not be representing him in any proceedings.

But union spokesman Ian Leavers defended the process to discharge an officer, saying criminal proceedings must be dealt with before employment could be terminated.

"It's just the way natural justice has to be done," Mr Leavers said.

"This is the most serious of crimes, it has been dealt with by the court and Mr Randall has not been represented by the QPU.

"I have no doubt the Queensland Police Service will act very swiftly."

Randall was sentenced to nine years in jail on Friday. He cannot apply for parole until he has served five years.

The Supreme Court was told Randall lied for four years about killing his son. ( ABC News )

His prison time will be spent in solitary confinement because of his history as a police officer and because of the nature of his crime.

The Supreme Court in Brisbane heard Randall "forcefully punched the defenceless 10-week-old" so hard it caused fatal abdominal injuries, in an attack at the family home at Victoria Point on Brisbane's bayside in 2014.

Justice Peter Davis said Randall lied for four years about what happened, claiming the injuries were the result of CPR.

"It is obvious that once you had delivered the blow to the defenceless baby's abdomen, his death was inevitable," Justice Davis told the court.

"His liver was shattered, other organs had been severely damaged and his aorta ruptured ... no medical treatment could save him," Justice Davis said.

The court heard Randall's wife and daughter were at the shops at the time and it was the first time the baby had been left alone with his father.

Problems at work and extra marital affairs were blamed for Randall feeling extremely frustrated and delivering the "spontaneous" blow.