Public Services Association (PSA) general secretary Anne Gardiner said a lack of staff meant case workers could visit only a fraction of children at risk of serious neglect or violence.

The outcry by the PSA follows a weekend media report of a Wollongong toddler whose death was being treated as a possible homicide.

The boy had been reported to the department three times in the five weeks before his death last August.

He had been reported to DOCS nine times during his 2½-year life for injuries including severe bruising and human bite marks.

Ms Gardiner said: "Our members report [case worker] vacancy rates of 40 per cent in some offices in western Sydney and regional areas, with a state-wide average of around 25 per cent.

"Only about one in 10 children assessed to be at serious risk are receiving face-to-face visits from case workers.

"It is extremely distressing to child protection workers to know they are only touching the surface."

She said PSA members wanted vacant case worker positions filled to ensure vulnerable children were seen by DOCS workers.

A department spokeswoman said case worker numbers changed all the time.

"Vacant positions arise when a caseworker leaves the organisation, accepts another role within Community Services or resigns, and the position remains unfilled while a new candidate is appointed.

"The Department of Family and Community Services does not publish statewide data on case worker vacancy rates."

The spokeswoman said the December Auditor General's report to Parliament showed vacant case worker positions at their lowest point in years, with 152 vacancies as at June 30 last year compared to 497 in 2010.