Can I use share portfolio losses to offset other income in Australia?

Capital losses from shares can only be used to offset capital gains, not ordinary income. If you sell shares at a loss, that loss is a capital loss. You cannot apply it against salary, rental income, or other assessable income. It can only reduce capital gains you make in the same or future income years.

If you have no capital gains in the year you realise the loss, the capital loss is carried forward indefinitely. There is no time limit. You apply it against future capital gains whenever they arise. There is no option to choose to defer the loss; it must be applied against capital gains as soon as you have them (subject to the ordering rules).

The 50% CGT discount applies after you reduce your gain by any capital losses. So if you have a $20,000 gain (12-month asset) and a $5,000 carried-forward capital loss, your net gain is $15,000, and the discount reduces it to $7,500 of taxable income. There is no wash-sale rule in Australia, though the ATO does scrutinise same-day sell-and-rebuy transactions and can apply Part IVA (general anti-avoidance) if the arrangement lacks genuine commercial substance.