I am not in QLD but the nights do get sticky here in north/west Sydney and I do use sleeping bags for DD. Still do. I use grobags for her and we buy them off the website. I use the 0.5 tog ones (which are almost like a sheet really) for summer.

I didn't use grobags untill DD was 6mths old though, but when she was younger I use to have her in a short sleeved romper in summer and have her wrapped in a muslin wrap. If it was a cooler night or the night got cooler I would have a light sheet or blanket handy to put on her if need be.

Could you have a fan handy for if it is really hot? You can get those small desk top ones. Don't have it blowing on her but have it in the room to circulate the air. Even have the window open slightly (if you are comfortable with this, or put a rod between the wall and window so the window can only be opened so much).

Hope that helps a bit

ETA: I found that alot of it was trial and error. Every baby is different with how easily they get hot/cold and it's just a matter of trying a few things out to get it right for your baby.