HECS & Salary Sacrificing

Hoping someone can help me here who knows more about tax/accounting than I do?

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Basically I earn a base salary of $61,000. I salary sacrifice a total of $18,550. I have a HECS debt of $46,659.55. Because of salary sacrificing, it bought me under the threshold to start repaying the debt, which I stupidly didn't notice until I got slapped with a tax bill.

My question is: how much should I be paying per week towards my HECS debt? I have calculated around $35/week, but not too confident in my ability to calculate all this.

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Thanks!