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If a professional person has been negligent in carrying out instructions, they owe duty of care and if the breach of the duty of care causes losses, then the person is liable for the losses. In this case the interest was incurred because he did not apply the correct category for tax, and therefore would be liable. If you have to borrow to pay the tax he should pay the costs of borrowing
How can we make him liable? Is this done at the small claims court?
The first thing would be to challenge him and ask what he is going to do about the problem.
If he denies liability then the DisputesTribunal is the place to resolve this
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