Would it not have been possible to call forward an FA from a slightly later flight and then call out a Standby to cover them?
Pacific Blue have I think one flight a day into Melbourne. AKL and CHC are the only crew bases and they don't usually overnight crew in MEL. Standby or other crew simply not avialable.
Circumstances for the crew (flight, cabin and ground) can be difficult when a crew members goes mid-duty so close to departure.