Have you considered diving the HMAS Brisbane off Mooloolaba?
They have dives every weekend and it is an easy drive from BNE and no restrictions on the flying.
Day diving from Cairns is not worth the money (IMO) as the reef is over dived and you will be in a group of about 20.
Another option is to go south to Julian Rocks. There are dive shops in Brisbane that do trips on some weekends