Some people have been asking about the weather in Melbourne in late-January in terms of what to wear for Linux.Conf.Au.
It is probably impossible to predict weather for a particular day this far ahead. But predicting a range for the week is not difficult.
I think that you should expect at least one day that is really hot with a peak of 37C or more and reasonable humidity with a possibility of another two days the same or similar. A day with a peak of 42C or more is not unlikely over the course of a week.
You should expect a range of temperatures, one or two days that are reasonably cool with a maximum of 25C would not be unexpected. Some heavy rain in short bursts is a possibility (based on the past few weeks – prior to that there was little rain and it’s possible that there may be some time without rain again), there is probably no need for a rain-coat if you have the option of waiting ~30 mins for the rain to pass before going outside. I expect that if there is any rain at a time when conference delegates are about to go out somewhere then things will be delayed.
I suggest that you wear jeans while on the plane but expect to wear shorts for your entire time in Australia. A t-shirt is a reasonable option but if you plan to be outside much then wear a long-sleeved shirt. As I don’t expect to be doing much work in traditional offices in the near future I’m wearing business shirts when I go outside, long sleeves with a collar is good for protecting against sun-burn and as they are light they keep me cool (t-shirts are tighter and thicker and keep you hot). However when at LCA I will be wearing t-shirts that I designed (which should be well suited to being inside and I don’t plan to do much outside during that week).
A Scott e Vest  is a good thing to wear. It has heaps of pockets for your electronic gear, is reasonably light, and can be worn on top of a t-shirt. The Scott company also sells a TEC shirt which is a long-sleeved shirt with plenty of pockets. I’ve had a Scott e Vest for a number of years and I might have to get myself a TEC shirt.