The Liquor, Hospitality and Miscellaneous Workers Union said the incident was caused by critical staff shortages in the sector and was no surprise to members.

The union's child-care organiser, Yvette Berry, said child-to-staff ratio shortfalls were not uncommon in Canberra, and the Government needed to shift its focus from waiting lists to improving conditions for workers.

''In the ACT, the turnover for staff leaving the child-care sector is 47 per cent, which is really big given that nationally it is 30 per cent,'' she said.

''It is a real problem and the reason people are leaving ... is because of the low pay and the lack of recognition.

''There is a mismatch between the remuneration and the responsibility in the work that you do.''

The Government said only a ''very small number of centres'' exceeded child-staff ratios during inspections last year. An exact number could not be provided.

Breaches related primarily to staff rosters not adequately covering the beginning and end of the day, or insufficient staff to cover staff breaks.

A spokesman for Youth Minister Andrew Barr said the incident at Garran was different because child-staff ratios could not be met across the whole day.

Parents who dropped their children off at the Garran centre yesterday morning were shocked some children had been sent home the previous day, many only finding out through the media.

It is understood at least 10 parents were asked to collect their children because several recent resignations led to inadequate child-staff ratios.

Raelene Kennedy, whose son, Alex, 2, has been at the centre since December last year, said it was ''alarming''. ''At the moment I'm not working but I feel for those parents that are,'' she said.

Parent Jenny O'Donoghue said the centre had asked her to pick up her daughter, Breanna, 2, but said ''okay'' after learning she was at work.

A letter about the shortages was handed to parents as they dropped their children off yesterday.

''We are working hard to appoint permanent staff members to replace these staff ...'' it read.

''We hope to be able to appoint these staff members if suitable to permanent positions this week.''