Capital gains, page-22

$100,000 capital gain on shares

you had a capital loss of $25k

you choose what gains to offset this against

in this case you only have the $100,000 capital gain to choose from

that brings it to $75k

you can now only take 50% of $75k

in other words, you get to choose capital gains you offset a capital loss against.

if you have the option, always choose to offset your capital loss against capital gains that do not come with the 50% CGT discount as this will end you up with more money.

in your case, you don't have a choice.

if you had $50k capital gain for stock CBA (no CGT discount coz held for 3 months) and $50k capital gain for stock NAB (entitled to CGT discount because held for 15 months), then you should offset the loss against the CBA stock first because youll end up with more money.

you get to choose what to offset the loss against

its described very well here: https://www.ato.gov.au/individuals/capital-gains-tax/calculating-your-cgt/