A former Queensland police officer has been sentenced to nine years in jail for killing his baby son.

Colin David Randall punched his 10-week-old son Kye Braxton Randall in April 2014, out of "frustration".

Justice Peter Davis today dismissed Randall's late plea of guilty to manslaughter, saying it was not proof of "genuine remorse".

Randall was originally charged with murder, after an 18-month investigation led detectives to believe Kye had injuries consistent with a fatal blow to the stomach. The court heard the punch had “pulped” his liver, and severely damaged other internal organs.

“It was sickening to think what happened to him… He was a 10-week-old baby and totally innocent of the world,” Justice Davis said.

Randall had been left to take care of Kye while his wife went to the shops. While they were out, Randall claimed he was vacuuming the house. But at some point, as his son sat in a rocker, he punched him in the stomach.

Randall called his wife telling her that their son had gone "limp, lifeless and that he was not breathing". Then he called Triple Zero for help.

But Randall lied to paramedics, not revealing that he had punched Kye. He kept that lie up for nearly four years, despite later being charged with murder, repeatedly telling his wife, family and investigators that the injuries were as a result of him incorrectly performing CPR.

At one point he told his then wife, "I was in a panic trying to save my son’s life. I was pushing down on his stomach. He’s a baby, I am 115 kilos… he didn't stand a chance".

A few days out from the start of his murder trial, Randall entered a plea of guilty to a lesser charge of manslaughter. Randall later blamed his reasons for striking Kye, on an "intense frustration" he was feeling at the time.

He told investigators his wife had been suffering from the flu, that he had been bullied at work and he was stressed from two affairs he was having at the time. But Justice Davis dismissed those reasons in his sentencing.

"Your explanation for striking Kye is unconvincing… You were a mature, well-educated man of obvious intelligence who was holding down a responsible job as a police officer,” Justice Davis told the court.

"The 'intense frustration' said to explain the blow really boiled down to normal domestic pressures of looking after children… quite frankly, there is no real evidence to suggest that they were particularly significant and the pressures caused by your extramarital affairs are matters of your own making."

He said Randall was only motivated by self-preservation.

"Your plea of guilty was late. The plea was made in the hope that it was the best outcome," Justice Davis told Randall as he stood in the dock.

"Your motivation to plead guilty to manslaughter… They’re motivations of self-preservation. You have opted to avoid a life sentence of imprisonment."

Randall was sentenced to nine years in jail for killing his son. He will be eligible for parole in January, 2021, taking into account the 832 days he’s already spent in prison.