A good sleeping bag lasts years. I got a Macpac Neve in 1992, I still have it and it is my favourite sleeping bag out of my small collection.
The old Macpac Neve it comes with probably a1/2 to 2/3rd down on top and a sleeve to slide your Thermarest or similar mat in on the bottom. At one stage Kathmandu had a similar style. I think that the down on top sleeping bag style was not favoured by some due to some customers complaining that they could not move around as much these types of sleeping bags however I find it great, it is not a snow bag though for that I probably prefer a full down mummy style. The down on top bags are great for opening up and just using as a blanket. Feathers on the bottom don't add to the insulation properties so you get a sleeping bag with probably a 1/3 or more less down and consequent weight saving and more compact.
Luckily my husband has a Macpac Neve, with a zip on the opposite side. Apparently there was an unpublished code to sell right hand zips to females and left hand zips to males ...
If you want a cheap sleeping bag "Roman" brand is, or maybe was in the past, quite good quality for the price and probably OK for coastal Qld. I bought a couple of Palm IV polyester filled bags quite cheap, they are good however the reality is that I choose a down bag as it is still smaller, lighter and warmer for the same weight and size. The Palm Iv is about the same size and weight as my trusty Neve. The advantage of polyester or synthetic is that it is not such a disaster if it gets accidentally wet. I always line a dry bag with a garbage bag and then my stuff it in and make sure the garbage bag is kind of sealed for sea kayak camping and wet overnight canyoning trips. If my sleeping bag has been a bit wet due to heavy condensation or rain, usually at the toe end, I have been lucky enough to get it dryish during the day.