The New South Wales Government is preparing to introduce laws that would allow babies to be seized at birth if their mothers abuse drugs during pregnancy.

Pregnant women who use drugs or alcohol will be made to sign Parental Responsibility Contracts ordering them to undergo treatment, or risk losing their child.

The contracts are already in use but currently can only be applied after a child has been born.

The Family and Community Services Minister Pru Goward says hundreds of heroin-addicted babies are born every year, with terrible consequences.

"So Parental Responsibility Contracts... extending those to before the birth means we have the opportunity to require a mum to go to a drug or alcohol abstinence program (to) manage her addiction to ensure that the baby has a much better chance of being born normal and she has a much better chance of keeping her baby," she said.

The Government has been repeatedly accused of inadequately resourcing case workers, but Ms Goward doesn't believe the changes will add the workload.

"This way we have a greater opportunity of intervening earlier.

"We have drug and alcohol services that are available that are provided to the mothers and in the long run this is a much more effective way of managing the baby and the resource allocation need not be so intense,"she said.

But the New South Wales Opposition is giving no guarantees that it will support the legislation.

Opposition spokeswoman Linda Burney says drug abuse by pregnant women is a serious issue but she is concerned about the details of the legislation, including other matters such as adoption.

"The timeframes about moving to adoption are very short.

"There is less emphasis on getting consent from parents for adoption and there is (a) possibility, as I understand it, that adoption will move from the Supreme Court.

"Those things do concern me and Labor will be looking at it very carefully," she said.