Capital Gains Tax, page-23

Hmm... I am not an accountant - however my understanding was that the (long-term) "more than a year" and (current) "less than a year" trades are calculated separately - so I'm not sure that your first calculation is correct there (didn't check the others).

Here's how I believe it works:
A. Calculate the current profit/loss on trades that are held less than a year. This is essentially "income".

B. Next - you calculate long-term profit/loss on trades that are held more than a year. If this is profit, then divide by 2 (the 50% CGT discount). This gives you your "long-term" profit. I am not sure if long-term losses also attract a 50% discount - it is a capital GAINs tax discount, so I think not, but hey, IANAA - check with your accountant

C. Finally, you need to take into account any carry-forward losses from prior years.

A + B - C = your taxable profits (if positive) - or your carry-forward losses (if negative).

Your taxable profits are added to your taxable income and treated as income in this financial year.

So - in your scenarios, I suspect it would be:
Note: (Numbers in brackets) denote negative values
1. A = ($1000), B = $500. A+B = ($500). You have $0 trading income and a carry-forward loss of -$500 for next year
2. A = $1000 - $1000 = $0. No trading income to report.
3. A=($1000), B=+1500. You have $500 of trading income for this year
4. A=$3000 - $1000 = $2000. You have $2000 of trading income for this year
5. A=$1000 - $1000 = $0. B = $500. You have $500 of trading income for this year.

In all cases, your trading income is added to your PAYG and other income and tax is calculated according to the income tax tables.

Again - I am not an accountant, just been doing this a while and this is my understanding of how it works - happy to be corrected.

Hope it helps