A 30-year-old man has been found guilty over the drowning death of his autistic, non-verbal daughter in the North Queensland city of Townsville.

The man, who cannot be named, pleaded not guilty in the Supreme Court in Townsville to the manslaughter of his three-year-old daughter and exposing his two-year-old son to danger on December 4, 2021.

The trial was told the children left through an unlocked front door late in the afternoon as their father slept after being awake for 31-hours on methamphetamine.

The girl's body was discovered that night in a stormwater drain less than 150 metres from her home.

Court hears both parents drug users

During the trial the court heard the the girl's mother and father were using and dealing methamphetamine in the lead-up to their daughter's death.

Their relationship had ended but he was staying at her house.

On the day of his daughter's death the man had been awake since about 10:30am the previous day. 

Text messages show he had stayed awake overnight using meth and messaging people about drugs while the children's mother was out.

The mother said the man was awake when she left the house about 5:35pm, but when she returned just under an hour later he was asleep, the children were missing, and the front door was wide open.

Nearby residents found the little boy wandering the streets in a nappy, and his three-year-old sister was pulled from a stormwater drain about 8pm.

Death of a child 'difficult matter'

In his closing address, defence lawyer Darin Honchin urged the jury not to think this was a drug trial despite having heard about the man's frequent drug use.

He said this was a father who had lost his daughter.

"The death of a child is always a difficult matter," he said.

But he said it was possible the little girl had managed to open the door herself though flicking the latch. 

He said his client had latched the security door, but not been able to lock it as he didn't have a key. 

"It's not beyond the realm of possibility that [the daughter] opened the door," he said.

Mr Honchin said it was not wrong for his client to have gone to have a sleep while his children napped.

His client maintained he had flicked the door latch to lock it.

"What more reasonable things could he have done?" Mr Honchin said.

The man gave evidence in his own defence, saying he saw nothing wrong with having a sleep while his children were asleep.

"I believe when kids go to sleep you can go to sleep as well," he said.

Prosecutor says death entirely preventable

But crown prosecutor Monique Sheppard said the father's claim he locked the door was a lie. 

Ms Sheppard accepted the father was genuinely distraught over the loss of his daughter.

But she urged the jury to put aside any sympathy for the man while they considered the evidence.

"We are dealing with the death of a little girl," she said.

She said the man's "selfish" drug use had impaired his duty to protect his vulnerable, very young children from harm, and he had been criminally negligent. 

"She died because of a series of decisions made by the defendant," Ms Sheppard said.

"Her death was entirely preventable.

"It was not an accident."