The decomposing and emaciated bodies of the toddlers were found in their cots on Monday by their 11-year-old sister, who smelled something unusual in the family's suburban home.

The twins had been dead for more than a week, investigations showed.

The children's 28-year-old father and 30-year-old mother were detained and charged with failing to provide the necessities of life after the mother allegedly told the police that she had noticed the twins were dead on either June 8 or June 9. She added that she had been suffering from a cold and rarely fed or changed them.

The father told the police that he did not know they were dead although he had walked past their room several times since their deaths.

The twins, named in media only as Lily and Zaide, weighed around 4kg when they died, about the weight of a normal newborn.

Their deaths have received prominent newspaper coverage and dominated television and radio reports.

'It's hard to comprehend that (something like this) can occur in our society, in a reasonably normal Brisbane suburb,' Abused Child Trust co-founder and chairman David Wool told local radio.

'It is a very important thing to realise that we should be looking after the next-door neighbour's kids, or the kids down the street, and keeping an eye on them,' he said.

A post-mortem examination was due to be completed and, depending on the outcome, prosecutors said charges against the parents could be upgraded to manslaughter or murder.

The sister who found the twins and her three brothers, aged three, four and five, are now in the care of their grandmother.

The lawyer for the estranged father told a Brisbane court on Tuesday that his client was not responsible for the care of the twins as he was rarely at home and his relationship with his childhood sweetheart had broken down six months previously.

'He hasn't had any contact with the children,' lawyer Michael Cridland said.

But police prosecutors countered that the man, a construction manager, still lived in the house and passed the room every day while going to work and taking the four older children to school and childcare.

Neither parent had a criminal or child welfare agency record, the authorities said.

Neighbours said one of the older siblings once arrived naked on their doorstep and asked to be taken in.

The couple's other children said they rarely saw the twins, who were reportedly confined to their room and rarely attended to for nappy changes and feeding.

The mother is to undergo a psychiatric assessment and face court with her former partner later in the week.

'The circumstance of the death of two 18-month-old children is bizarre, given that both the parents had been in the house for approximately one week since their deaths,' Court Magistrate Noel Nunan said.

REUTERS, AGENCE FRANCE-PRESSE