Hi Snow,
It's winter now, August is the last winter month. Spring starts in September, therefore summer is Dec-Jan-Feb.
Well, as a rough guide, Tasmania, Adelaide and Melbourne are the cooler states. Followed by New South Wales (Sydney) and Brissy (Gold Coast). W. Australia (that's Perth) can be really hot in summer months.
Sun is piercing in Australia during summer months, even in the cooler states. Do use lots of sunblock and bring a hat and your shades.
But all that said and done, Melbourne is famous for its four seasons in a day

So, do bring along a cardi for the sudden change in weather. If you like, you can give me your email address, I can give you a list of things to do in Melbourne.
As for Gold Coast, I've been there twice and naturally, you must go the MovieWorld and the beach. Buy your swim suits there, lots of variety! Most of what we get in Singapore are from down under anyway! And if you're into diving, wet suits are cheap here!
And for anyone doin DIY and are rather price-conscious, you might wanna consider using Autopia Tours for your day tours. There are a number of operators here, like Grayline, but Autopia is about half their prices. They use 22-seater buses, smaller but the drivers are great!
Is it DIY?