‘I cry for my baby boy every night': Queensland dad jailed for drowning infant son

More than three years after he plunged into a Queensland river cradling his baby son, David Paul Fisher is still haunted by his child's drowning death.

"I cry for my baby boy every night," he told Brisbane's Supreme Court.

The 42-year-old had taken teething baby Elijah out on the evening of June 23, 2012, hoping a walk would soothe him.

But when Fisher returned home, he was dripping wet, distressed and alone.

The father had fallen into the Logan River cradling his son in his arms, and let go.

The six-month-old's body was found downstream the next morning.

Fisher today pleaded guilty to manslaughter, as well as two drug-related offences.

Paramedics who attended that night found him "inconsolable", Crown Prosecutor Todd Fuller QC said.

He said Fisher had spent the morning taking drugs including synthetic cannabis, which he later told police he used because it "took away the pain".

These substances - including cactus powder - exacerbated underlying mental conditions and caused severe, psychotic delusions, the court heard.

But defence barrister John Allen QC said Fisher showed considerable remorse and has even earned the forgiveness of his now ex-wife.

The court heard the pair had adopted an alternative lifestyle and spent time travelling throughout Australia and New Zealand in a converted bus prior to Elijah's death.

"It's clear that the emotional consequences to a large number of people have been enormous. I acknowledge they have also been enormous for you," Justice Ann Lyons said.

She cited psychological reports which stated Fisher was at times "overwhelmed by grief" at having caused his son's death in the throes of an hallucinatory episode and was otherwise a loving parent.

Nevertheless, Justice Lyons noted the crown's submission young Elijah was extremely vulnerable when Fisher released him into the swirling river.

She sentenced Fisher to seven-and-a-half years' imprisonment, to be eligible for parole after three-and-a-half years.

Considering more than 1200 days already spent in custody, he could be released before the end of the year.