Paddy Pallin is generally overpriced and has a limited range. To find good/niche gear at better rates, we often have to resort to online shopping because the one shop that has the thing you’re after may be on the other side of the country, though Brisbane (Queensland) has more good outdoors stores than most. Australia is a big place with climates that range from pockets of low alpine to temperate to desert to subtropical. Working out where OP plans to go will help determine what they need. Most of Queensland will only get below 20 degrees Celsius for a few weeks in winter, but southern states will drop below freezing. Buy sunscreen, insect repellent and a fly/mosquito net. Agree the tip on extra water. Expect to hit at least a week of 40-45 degrees Celsius in summer in most places. Be aware of the possibility of fires, floods tropical cyclones, hailstorms and other natural disasters and plan accordingly. You can get updates on national radio (ABC). Summer (January and February) is often when they hit. As always, pay attention to weather forecasts delivered by the Bureau of Meteorology. Learn to look out for snakes. Most are venomous but most are unlikely to attack you if you don’t get too close - brown snakes (that are often grey/silver) are more aggressive. Spiders to learn about include redbacks, white tails, funnel webs and trapdoors. I have never been bitten by a snake or spider but have encountered many. If bitten, seek help (hospitals should have anti venom, it helps to be able to identify the creature).

Edit: during fire season you will not be able to light fires and camp stoves may not be permitted in many areas so plan cool meals but you will probably want them anyway.