I assume that they were used for business purposes and that is why you were depreciating them. The tax debt is the difference between the written down proceed and what you sold them for. This is closed as profit.
During the depreciation period that amount was deducted from your income and you did not pay tax on that amount.
The cars are usually depreciated over a period of 8 years maximum depending on the method used. Here is some information.
If he depreciated them at 25% then the full value was deducted in 4 years.
You signed the returns and you give them instructions. if you are saying that he has acted on his own then you can complain about him. But you cannot get the benefits twice. If he redoes the last 4 years of returns you have to pay tac on depreciation and then you can avoid paying it now.
Either way you will have paid tax to the ATO.
I hope this makes sense and is of assistance. If there is nothing further
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