There's more to this than a shortage of charter aircraft. A few weeks ago Tim Nicholls was on the news demanding that an election be called and to stop messing around with the date. Well an election was called and they were caught short and had nothing organised and of course they want to do it on the cheap.
On top of this, the carriers which used to be predominately charter are now operating RPT, so there is a shortage of SAAB's, EMB135's and F70's to go around. I also don't think that REX has anything spare because apparently they are struggling with crew to conduct their own scheduled flights.
Also, since the last election the B200 and Chieftain charter aircraft at Brisbane Airport are almost non-existent.