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The family of a nine-month-old girl whose body washed ashore on a Gold Coast beach "sacrificed" the baby by throwing her into the water in New South Wales, police in Queensland believe.
Sources familiar with the investigation into the death have told AAP the family of four were tracked on CCTV travelling from the Gold Coast to the Jack Evans Boat Harbour across the border in Tweed Heads on Saturday.
It's alleged the naked baby was thrown into the harbour and her empty pram abandoned alongside the waterway.
Queensland police then captured images of the now trio catching a bus at Coolangatta and returning to Queensland, sources said.
The baby's body is believed to have drifted around 30km in the current for two days before washing ashore at Surfers Paradise beach where she was found by passers-by around 12.30am on Monday.
Queensland Police confirmed in a statement on Tuesday night that the baby's death occurred in NSW but gave no further details.
On Wednesday, NSW Detective Chief Inspector Wayne Walpole told reporters in Tweed Heads police were using divers in the Tweed River and the Jack Evans Boat Harbour.
"Clearly the child was located in the water up at Surfers Paradise, he said.
"We want to know how the child got there. We are looking for evidence that could assist us."
The family were familiar with the area, locals say.
Tweed Heads watersport business owner Trev Arbon saw the family in the park on-and-off throughout the last week.
"It was all really strange seeing them walk around because they had the little babies with them," he told AAP on Wednesday.
"It's just a tragedy for that beautiful baby - really sad."
Mr Arbon said the family wasn't from the area and kept to themselves - even when others reached out to offer them money for groceries and nappies for the children.
The father's hearing to be extradited to NSW was delayed in Southport Magistrates Court on Wednesday because he faces other charges which must be dealt with before the extradition can proceed.
He appeared wearing a white hospital gown and was represented by Sunil Dutt of the Aboriginal and Torres Strait Islander Legal Service.
The 48-year-old is yet to be formally charged over the death.
Police sources in Queensland allege the mother admitted to knowing about the planned "sacrifice" - as it has been described to AAP - but only the father would be accused of throwing her into the water.
It's believed the 23-year-old mother won't face charges, has been released from custody and placed in the mental health system in Queensland.
Queensland detectives have been "deeply distressed" by the circumstance of the child's death, the sources said.
Queensland Premier Annastacia Palaszczuk said there would be questions asked of child safety departments in both states.
"I think we need to recognise that homelessness is a big issue," she told reporters.