A State Government decision to not put a magistrate on duty yesterday put police under pressure and was "inexcusable", the police union said.
Police officers were told yesterday morning the Perth Watchhouse, which can hold 50 prisoners, was full and they would have to monitor prisoners at suburban stations.
Acting Deputy Commissioner Gary Budge said the situation took officers from frontline work and affected their ability to respond to calls.
Mr Budge and WA Police Union president George Tilbury said with New Year's Eve the busiest night for police across WA and the potential for trouble at the Perth Cup, a magistrate should have been rostered to work.
An Attorney-General's Department spokesman said it was a "long-standing arrangement" that courts did not sit on a single public holiday because lockups had enough capacity.