marking down 'Closing Stock' at 30 June only applies if you are 'Trader' ('Business') for tax purposes
say at 30 June 2018, your Closing Stock included 300,000 AVZ shares bought at 3 cents and 200,000 AVZ shares bought at 25 cents each.
at 30 June 2018, the closing (market) price of AVZ was 10.5 cents
to minimize your taxable income, you value your 300,000 AVZ shares at 3 cents each (cost price) and you value your other 200,000 AVZ shares at 10.5 cents each (market price). this lowers your taxable income.
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however, as I warned, this potentially gives rise to a larger tax bill the next year if you have a bad year next year
for example, i just lodged my tax return for FY2018 (to get a refund on PAYG instalments paid for Sept 17, Dec 17 and March 18 quarters). i got hammered in the 2nd half of FY18, losing much profit i made in the 1st half. 40% of the taxable income i lodged for FY2018 was the closing stock i marked down at 30 June 2017
40% of the tax i must pay for 2018 is from profit made in 2017
