The top marginal tax rate on income would be no more than 44%, including medicare levy. 50% tax is the threshold where you go from working for yourself and giving a share to the goverment to where you work for the government and get to keep a share.
If your marginal income tax rate is 45%, then an extra $100 earned before tax will leave you with $55 after income tax is deducted. With that you could buy something that costs $50 before GST. So a 45% income tax rate combined with 10% GST is effectively a 50% tax rate.
Looking at a 45% income tax rate another way, if you provide a service for which your customer pays $110, you first need to deduct $10 in GST. That leaves you with a taxable income of $100. You pay 45% income tax and you are left with $55 which is 50% of what your customer paid you.
A top marginal tax rate of 44% would therefore ensure that tax is less than 50% once GST is taken into account.