The currency is determined when you set up the supplier.  You cannot change it.   You’d have to create a “new” supplier but this time select AUD as the currency.   Put AUD as part of the name, as you can’t have the same name again.

Resolution for 'AUD issue: 'Can you change the currency that you pay a supplier in? ie from USD to AUS' available: Yes.

The AUD issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the AUD issue yourself by looking for a resolution described in the article. If it is a complex AUD issue or you are unable to solve the issue, you may contact us by clicking here or by using other AUD support options.

For more information about resolving issues related to AUD in Quickbooks/QBO or to get support for the issue 'Can you change the currency that you pay a supplier in? ie from USD to AUS', click the link below: