A meth-using father who told his baby son "sorry" before abandoning him on a boat ramp south of Brisbane 12 years ago has pleaded guilty to manslaughter, with the child's body never found.

Key points: The father of Baden Bond admitted to leaving him at a boat ramp

The father of Baden Bond admitted to leaving him at a boat ramp The 22-month-old's body has never been found and originally vanished in 2007

The 22-month-old's body has never been found and originally vanished in 2007 The family was known to the Department of Child Safety

Shane Arthur Simpson, 51, was originally charged with murder but on Monday pleaded guilty to the lesser offence of manslaughter after a new indictment was presented to the Supreme Court in Brisbane.

The child's mother, Dina Colleen Bond, 45, pleaded guilty to accessory to manslaughter because she was not at the boat ramp when the child was abandoned, but tried to protect her partner from the authorities.

Prosecutor Danny Boyle said Simpson abandoned his 22-month-old son at dusk near a boat ramp at Logan in May 2007.

Baden Bond was last seen at a property in Woodridge, south of Brisbane. ( Supplied: Queensland Police Service )

"Baden Bond has never been found and therefore no cause of death has been established," Mr Boyle said.

The prosecution said the toddler's parents referred to him as the "devil's child" and "evil" and moved to New South Wales a few months after they deliberately left the child at the boat ramp.

"[Simpson] told police what he did was not murder. He left him somewhere," Mr Boyle said.

"No-one wanted Baden and he was tired of responsibility."

The prosecution read out parts of Simpson's interview with police after he was charged with murder in 2017.

The court heard Simpson sat on a park bench with his son for 15 to 30 minutes before patting him on the head, saying "sorry" and leaving him at the boat ramp.

"I didn't even look back in the rear-vision mirror," Simpson told police.

Mr Boyle said Simpson also told police: "I've had to live with this for 10 years.

"It's like he knew I was going to leave him there.

"The look in his eyes, I still see the look in his eyes."

Simpson's defence counsel said Simpson had a long-term addiction to methamphetamine that was "particularly intense" in 2007 when he abandoned Baden.

Flowers and teddy bears left outside a Woodridge house in 2016. ( ABC News: Casey Briggs )

Family had history of hostility with child safety department

The court heard Bond and Simpson had a history of hostility with the Queensland Department of Child Safety, who took Baden into care just after his birth in 2005 because the baby had drugs in his system.

The toddler was released back into his parents' care in October 2006.

The prosecution said Centrelink was advised in May 2007 that Baden was no longer in his parents' care but no notification was made to the department.

The court heard Bond and Simpson told officers that Baden was staying with relatives, when they came to do checks at the home and the child was not there.

Department officers drove past the house again in August 2007 but it was locked and vacant because the pair had moved to NSW.

The court heard the NSW child safety department began focusing their attention on the missing child in September 2015.

Simpson told them Baden was travelling with relatives around Western Australia.

Mr Boyle said when NSW police came to the parents home in March 2016, Simpson told them: "I gave him to someone, it's none of your business … I don't have to tell you, to who."

The pair will be sentenced at a later date.