That's a really tough situation, tax on that type of gain is...

That's a really tough situation, tax on that type of gain is going to hurt (regardless of whether ITP have calculated it correctly), especially as your capital is tied up.

You definitely can't use future cgt losses against past cgt gains, cgt doesn't work like that.

Ideally you would have sought tax advice on the initial gain, then you could have set that aside.

Assuming you're still working and accumulating super your super guarantee payments are going to attract a small amount of Div 293 tax just to top that situation off, so remember to elect to have that taken out of your Superfund rather than add to your current predicament.

Once lodged you could opt into a payment plan with the ATO, it's likely in your situation they will remitt any interest as long as you make the payments and explain the dire situation, I'd wait to see if they start applying interest before calling.

If you hold off and lodge late the ATO will most likely immediately apply backdated interest to the outstanding debt once you get around to lodgement.

Hopefully for you AVZ come good, or at least good enough to repay the debt.

The above is not financial advice and shouldn't be taken as such.