So your 8 year old would be turning 9 in March 2011? Then they'd most likely be going into Year 4. If your 6 year old is turning 7 in mid July 2011 they could be in Year 2 or you could opt for him to go into Year 1.
In NSW it goes like this:
Preschool (optional, play based) start the year you turn 4 (if before cut-off).
Kindergarten - first year of school, start the year you turn 5.
Year 1 - the year you turn 6 and so on up the grades.
All, of course, based on a pre-cutoff birthday.
However, there's lots of flexibility in terms of when your child starts. Some people would opt to hold a March born child back a year too. My DS is a Feb baby and is one of the youngest in his year.
However, you might find that the content is easier in the younger years here and that holding back would lead to boredom. School in Aus in the early years also sounds much more relaxed than similar years in the UK, so you may well find that both your children cope really well with their correct years here.
There's also the option to repeat a year here, a decision usually made on social and emotional rather than academic terms as differentiation should mean all academic levels are catered for.
Interestingly, given the purported choice in education in the UK, parents seem to have a much greater say over their child's education here. We can decide, within an 18 month frame, when our child starts. Schools are not closed, and never locked, and parents are much more welcome in the classroom. Outside of a few, densely populated areas, catchments are pretty big and unless a school is totally full (pretty rare) you can choose to go to another local school however, most schools are pretty similar so it's pretty unusual for children to travel to school.
It's highly unlikely that there'd be anyone around and available in late December/early Jan as it's right in the middle of the summer holidays. However, the school you are in catchment for has to take your children, even if they are full, so you are not at risk of not getting in anywhere. If you leave a message, and email, you may well find that someone from the school contacts you during the holidays anyway. If you know where you'll be living, then you can probably sort much of it on the phone and via emails before you arrive anyway.
Good luck with the move, and feel free to ask any questions!