I’ve had extra tax withheld because I didn’t provide my TFN but I’ve updated it now. How can I get the tax back?

You will need to make this request to the ATO. Information is available on the ATO website: https://www.ato.gov.au/businesses-and-organisations/international-tax-for-business/in-detail/income/refund-of-over-withheld-withholding-how-to-apply