An Australian expert in post-natal depression has said mental illness or desperation might explain why some mothers may abandon their babies.

A newborn baby survived for six days at the bottom of a stormwater drain in Sydney before being discovered on Sunday.

Police have charged the baby's mother with attempted murder and say it is unclear how the baby survived for so long, apparently without food or water.

Bryanne Barnett, clinical director at Raphael Centre Blacktown, said the abandonment of a baby may generally be explained by a variety of reasons.

"The woman may be ill or desperate for other social reasons," she said.

"There will be a reason. We may not agree with her decision about how to solve her problem. But there will be a very good set of reasons.

"It needs to be very carefully thought about.

"It is a miracle that the baby is still alive.

"Some babies have tremendous survival power."

Professor Barnett said people were often divided into two camps - they either felt sorry for the mother or were "very blaming" of her.

"It's important to keep a balanced view here. Something has to be far wrong before a mother does this and we need to know the circumstances and not move quickly to blaming or criticising her," she said.

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