People wear lots of different things and call them "raincoats". A raincoat for bushwalking is one which is actually waterproof. That is, you can wear it for several hours in the rain, and it will keep out all or almost all the water. Many items called raincoats will allow water through them if used in the rain for more than a few minutes. Cotton coats, unless properly waterproofed are not sufficient nor are thin "spray jackets".
Two things are obvious about good raincoats: (1) They are expensive (2) They are not particularly durable. Gore-Tex is still largely considered to be the best as long as you are not scrub-bashing, and as long as it's cool - in warmer weather they don't "breathe" as well as you might hope. They do work well in most Tasmanian conditions though, and keep out wind and rain when new. However they will develop pinholes in the medium term, and will start to admit water. Eventually the waterproof/breathable membrane will fail over large areas, for example under pack-straps. If you take the coat, say, every weekend, and use it on wet weekends, perhaps 1/3 or 1/4 of trips, you could expect to get several years' use as long as you didn't go scrub-bashing, and followed the manufacturer's care guidelines.
There are other good waterproof/breathable materials, of which Hydronaute used by Mont is one, which are also very waterproof and breathable. You should discuss it with the people in the shop.
Really, if you are going to walk in continuing rain, you need a coat made of fully waterproof material, which also has sealed seams, and has some design features to improve the ability of the coat to keep water out of openings. These last might include storm flaps on zips, closures at wrists and various ways of reducing the size of the face opening in the hood. You will also find that walking in the rain will normally be more comfortable if the coat is a bit longer to cover your shorts, or most of them. Coats on special are often of the shorter variety - perhaps they import too many and find they're not very popular in Tasmania. Some japaras will be sufficient, but others are only designed to get you home from the shops if you get caught out in the rain.
Waterproof trousers may also be useful at times, but are not nearly as essential as a coat. Unless it's really cold, you can get away with wet legs for quite some time, although if in a cold wind they can become quite uncomfortable.
More useful than trousers are waterproof gloves. There are various types, but most useful are probably a pair of overmitts. It is surprising how quickly uncovered gloves can become soaked in the rain, especially if you have to handle the undergrowth or rock at all.
For more information try the rainwear section on the Australian Bushwalking Website's FAQ.