"The mother of 18-month-old twin toddlers who lay dead in a bedroom for a week allegedly told police: "I don't think I fed them enough."

Their father had walked past the children's room but said he didn't know the boy and girl were dead, and hadn't seen them since Christmas, a Brisbane court also heard on Tuesday.

The children's deaths emerged after their 11-year-old sibling - alerted by an unusual smell - found their decomposed and emaciated bodies in the front room of the rented house in Brisbane's Sunnybank Hills.

The child allegedly said to her mother: "I know why you have been crying now."

Police found the toddlers' bodies at the couple's house around 7pm (AEST) Monday.

The 28-year-old man and his 30-year-old partner have been charged with failing to provide the necessities of life.

But the charges could be upgraded to murder, Brisbane Magistrates Court was told.

The couple was remanded in custody until Thursday pending the results of post-mortem examinations.

Police prosecutor Sergeant Tina Green told the court that once the examination was completed, it was "most likely" the charges would be upgraded to manslaughter or murder.

An earlier court hearing was told that police found the bodies in a "state of decay" and the children appeared to be malnourished.

The mother told police she had noticed the twins were dead on either June 8 or 9, the court heard, but she did not report their deaths.

She told police she had been suffering from a cold and rarely fed or changed the twins, it is alleged.

The court heard the children weighed the same as newborns and appeared to be suffering from malnutrition.

One weighed 3.6kg and the other 4kg, and the mother allegedly told police: "I don't think I fed them enough."

The couple's other four children said they rarely saw the twins, who had been kept in the front room for most of their lives, the court heard.

Defence lawyer Michael Cridland, appearing for the father, told a second of two court hearings on Tuesday that his client had only been informed of the children's deaths on Monday.

His client was "further removed in his culpability" than the mother because he had not been responsible for supporting the children.

Mr Cridland said the pair had been undergoing "significant relationship problems", and his client had not seen the twins since Christmas, despite living in the same house.

The mother was the primary carer and the father took the older children to school before going to work each day, he said.

However, Sgt Green said the man had to walk past the twins' bedroom to get to his own room, where he slept separately from his partner.

Both the accused refused to undergo a psychiatric assessment.

They are expected to make applications for bail on Thursday.

The mother's four other children - three of whom were fathered by the accused man - are staying with their grandmother.

A spokesman for Queensland's Department of Child Safety said the twins were not known to the department.

He said the family was reported to the department before the twins were born, but no evidence requiring action was found.

"Where there is no evidence for the department to take action, it has no legal basis to intervene with a family and it would be totally inappropriate for the department to attempt to do so," the spokesman said."

- thedaily.com.au