Industrial action?
Does it count as industrial action if we refuse to work any non-rostered hours for a week?
Possibly with the exception of someone going sick and someone being called in to replace - which would only be replacing a previously rostered shift and therefore something that we would normally be called in for and consider "reasonable" overtime.
Then the public and the parliament (and management) would see how understaffed the organisation really is on a standard week.
Yes it would be inconvenient, but it would prove a point without being too aggressive.
Just a thought.