You should really keep records of each trades realized profit as income, if the platform keeps bad records then you need to figure it out based on what records they have. Like do they tell you what leverage you were using. If your position is worth 3000 and your leverage is recorded as 3x, you know your underlying collateral was 1000 usd.
But yes there's no way to lose more money than you put in as collateral, as you can only be 100% liquidated.
So just ask yourself, how much did you deposit to the platform.
How much did you withdraw from the platform.
And how much collateral value did you have left in the platform EOFY.
If you lost everything you deposited, then you can find your deposit records easily on either the withdrawal side or the platform itself, and that is your total loss in the example of having no collateral left you never withdrew from the platform or somehow sold out to your bank. et cetera
Afaik, I'm not an expert in tax OR math, haha but thinking through it logically, and being familiar with the concepts of leverage.
PSA btw you want to look into volatility decay if you're holding leveraged positions for more than a day. It can be a killer. Price can return to your entry on the underlying asset but your leveraged asset hasn't returned to the same price it was when you entered.
Be careful with leverage it can get you in many ways.
I personally only use it to free up value I already have on hand, never to hold a position i'd never be able to afford. NFA, just you should apply as much caution to leverage as there is risk and leverage amplifies risk. Yet people approach it differently, the higher the leverage they take the more entertainment takes over and caution goes out the window.