not trying to scare you totz but do take note of your sis' address and phone number. and try to ask her to stay up till you have cleared immigration n customs. coming into aus with a one way ticket will be akin to painting your back with a big bullseye. make sure your english is up to scratch as well and is able to explain why you've bought just a one way ticket without being nervous as the immigration guys tend to spot n pick single people who appear like they are jumping aeroplane, and from watching a tv show called border security here, u might run into some trouble.

^ depends really. i've personally never experienced being busted by the immi peeps, but it depends on the mood or the personality of the peeps, UNLESS you look seriously dodgy or fidgety when they talk to you. i can share 2 incidents:

1. my best friend came down to adelaide to meet us. he got held at the immi cos he was travelling alone and didnt have a hotel booking so my other friend who was at the airport to pick him up had to go into the immi to clear him through. they literally came out to the arrival hall asking for anyone who knows the guy in question while pointing to the passport.
2. my sis' bf came down to visit her in sydney, and while transiting at goldcoast, he was held at the immi for having some work clothes and a resume, plus he did not have a hotel reservation. highly suspicious, no doubt, and the immi went through his belongings like a crazy kid on christmas morning. even as much as going through his sms'es on his phone. they then called my sis to verify before allowing him to pass.

and yes, they do have the time to check, as it IS their job to do so. and yes, they have an uncanny ability to pick out dodgy people, especially those travelling alone and not for studies.

http://www.bom.gov.au/cgi-bin/wrap_fwo.pl?IDN10064.txt

go to that site to check. nothing is more accurate that the bureau of meteorology's forecasts. they provide up to a week in advance, so you can plan ahead, but do be aware that the further the forecast, the more likely it'll change as the date becomes nearer.