Something for Option Holders to Consider, page-27

From the same document above, I think it's pretty clear.

CGT discount on shares or units you acquired from exercising rights or options

You can only use the discount method to calculate your capital gain from an asset if you own it for at least 12 months. In working out any capital gain on shares or units you acquire when you exercise a right or option, the 12-month period applies from the date you acquire the shares or units (not the date you acquired the right or option).