You don't pay your individual tax in September or October or November 2017 - especially if you got a tax agent representing you.
Via tax agents you can delay the lodgement and payment of your 2017 income tax til March to May 2018.
Makes absolutely no sense to sell now, in order to save to pay tax in March to May 2018 - more than 6 months away.
The only thing that would make sense would be his 2016 income tax where he owe money and had not paid the ATO, which would have been due in March to May 2017.
But you can easily sort that out via a bank loan or make arrangements to pay by instalments.
Its not like the ATO knocking down your doors to get their money.
So the question is why sell now when more results are due next week?!?