I'm reasonably happy with the Australian tax system, but I would make modifications.
Firstly, I wouldn't get rid of the GST. I'd just counteract its effect on income inequality with more steeply progressive tax thresholds.
Secondly, I'd up the rate of compulsory superannuation (if we're counting that as part of the tax system) to, perhaps 12% (up from 9%). And I would get rid of salary sacrifice. It's just a legalised tax fraud method.
Thirdly, I'd transfer all state taxes to the federal level. Ideally this would come with the abolition of the states as anything more than geographic groupings.
Fourthly, I'd crank up taxes on alcohol and tobacco, and on fuel, to a small degree. I would legalise most illicit drugs, and then tax them to hell and back.
And finally, income tax. This is what it is currently, with the marginal rates.
$0-$6000-----------> 0%
$6001-$35 000------> 15%
$35 001-$80 000----> 30%
$80 0001-$180 000-> 40%
$180 001+ ----------> 45%
And this is what I would change it to something like this:
$0-$12 000-----------> 0%
$12 001-$30 000------> 17.5%
$30 001-$60 000----> 35%
$60 001-$100 000--> 42.5%
$100 001-$150 000-> 50%
$150 001-$500 000-> 60%
$500 001-$1 000 000> 67.5%
$1 000 001+ -------> 75%
So, someone that earns the average tax payer, earning $A48 000 would currently pay $A8250 tax, whilst someone who earns $20 000 would pay $2100. Someone earning $80 000 would pay $17 850, and someone earning $150 000 would pay $45 850. Under my system, the average earner would pay $9450, and the $20 000 earner would pay $1400. The $80 000 earner would pay $22 150 and the $150 000 earner would pay $55 650. It would promote much better equality, IMO, whilst allowing for greater government spending and economic control.