Miss E - I was told by my employer and FAO that it is all taxed the same despite what you have previously earned. 15% is what it is, I am a high income earner and am being payed through employer and it is the same as it is through the FAO, that wouldn't be fair if it depended on how much you earned for the entire year, claiming it before July shouldn't make a difference, I got a big back pay of 9 weeks worth of it approx $5000 before tax and it was still only taxed at the low/weekly level. Briggsy's girl will know for sure but I worked full time up until I went on leave and in a high bracket and it hasn't made a difference in the amount of tax taken, my best friend had her baby a few weeks after mine and she is taxed the same as me despite earning less than half of what I did.
That may be the case when it is being paid to you, but when you submit your tax return you may find differently. Because your employer is paying it as a 'weekly/fortnightly' payment for a specific pay period they tax it as if that is your normal pay amount. When you submit your tax return and are accessed for the entire year you may find that you have a tax debt and need to pay more tax because your tax rate is more than the 15c in the dollar that you paid. That is why the PPL and BB comparison tool asks for your taxable income, because it does affect how much tax you have to pay on the PPL.

In terms of the dates it does make a difference and it did when I did the calculations on the FAO website. If I claim it in the 2011/12 year I will be in a much higher tax bracket (because of my YTD earnings) than I will be if I claim it in 2012/13, because I only intend to work for the last couple of months of the financial year. I can't remember the specific result, but the difference was significant.