Child welfare authorities intervened to protect one of Jules Mikus' children from known sex offenders late last year - but let the baby stay with his paedophile father.

Mikus, his partner Shirley Te Kooti, and their young baby were staying with a Porirua family in which there had been sexual abuse.

Social workers told Mikus last October that he and his family would have to move out of the house or they would remove the baby from his care.

Mikus and Ms Te Kooti moved out and were allowed to keep custody of the child despite Mikus' sex convictions dating back to 1973, and the fact that social workers had already taken children off the couple because of concerns about both of their histories.

The child was still living with them when Mikus was arrested in February for the 1987 rape and murder of Teresa Cormack.

After the 44-year-old's conviction this week for the Napier schoolgirl's killing, Child, Youth and Family announced a review of its involvement with Mikus and the children he had had contact with over the past 20 years.

Chief social worker Shannon Pakura confirmed that the department had known about Mikus' previous sex offending and had been notified of at least one complaint of sexual assault in 1986.

The department was also investigating incidents in 1992, 1993 and two in 1995 and says that it has fears about what may have happened to children in his care.

The Weekend Herald has learned the department's review will also include how social workers worried about the child staying with the Porirua family allowed him to stay with Mikus and Ms Te Kooti.

"The whole issue of what consideration was given by social workers to children remaining in Mikus' care is one of the key focuses of the investigation," said CYF spokesman Stephen Ward.

He would not comment on the individual case, however.

Mr Ward also confirmed that the department was looking at its contact with Mikus dating back to his first sex conviction as a 14-year-old and whether his victims were properly protected.

In later years, the department never placed children in his care but there are instances where social workers aware of his background allowed him to continue to have access to biological and step-children.

Dr Ian Lambie, a clinical psychologist at Auckland University, said research suggested that sex offenders were less likely to molest their own biological children than step-children.

But with someone who had committed as serious an offence as Mikus, it would be difficult to predict what else they had done.