Astral GoCardless - Help:

Borderless payments enable you to collect payments across multiple bank debit schemes and countries into a single bank account. The money will be converted each day from the customer’s currency it was collected in, to the currency your account is in and paid out into your existing account.

On your GoCardless dashboard you will see your home currency indicated by the little house sign in figure 1.

1

Figure 1 | Home Currency

Therefore, if an AUD payment collection is made, it will be converted to GBP and paid into your GBP account.