If you carry out an activity in "a business-like manner" then you can claim losses and expenses against income.

If you won $35k on the geegees, the taxman would likely let you keep it as a windfall profit. The flipside of that is that if you lost $35k on the geegees there's no way you'd be able to claim it as a business expense, since you need to keep some pretty serious books to be considered a professional gambler by the ATO.

Gambling on financial markets is a trickier business. You can bet that if you hung on to that $35k profit and didn't claim it as income, the tax office would eventually ask you some pointed questions about it. You could try to make the case that you're not trading in businesslike manner - you're essentially using forex like a poker machine. If your statements back up the fact that you're a miserable trader you'd have more luck convincing them that you're just a punter.

If you want to claim your losses, you need to show them that you're carrying out your trading in a businesslike manner. There's a ruling on similar types of markets: https://www.ato.gov.au/law/view/document?docid=AID/AID201056/00001