Hi @ChloeAnne,
Your sole trader business income is taxed at the 15% working holiday maker rate for the first $45,000. Your tax agent's information isn't correct.
When you're an employee, your employer withholds tax at 15% if they're registered as a working holiday maker employer. As a sole trader, no one withholds tax from your payments during the year. But when you lodge your tax return, your business income gets taxed at the WHM rates. That's 15% on the first $45,000.
The difference is the withholding, not the final tax rate. Employees have tax taken out during the year. Sole traders pay all their tax when they lodge their return.
You need to lodge a tax return regardless of how much you earn as a sole trader with an ABN. When you lodge, your income will be taxed at the correct WHM rate of 15% on the first $45,000.