Hi,
No, to be eligible to start Primary School in NSW children turn 5 on or before 30 July of the year they start i.e. they start in Jan 2012 and turn 5 before 30 July 2012. The first year of Primary School in NSW is called Kindergarten. So, if you arrive in May your daughter will be in Kindergarten, she will be joining part way through the year, she will move into Year 1 in January 2013.
It's worth mentioning that by NSW standards your daughter will be young joining Kindergarten aged 4 (nearly 5). In NSW children don't have to attend school until they are aged 6, many families wait until children are 5 (nearly 6) for them to join Kindergarten.
We're from the UK and started our son 'young' (4.5) and it has worked out fine but it I was worried how it would turn out.
Are you aware that you have to pay public school fees in NSW on a 457 visa? Currently $4,500 per year for Primary School (plus uniform etc).
All the best with your plans.