So following on from this thread on Monday, I was able to successfully apply the referral promo code (and it's been noted under History with "Introduce a friend - credited $20" at the time of activation to my $40/40GB sim.
However, when I go to Payment Summary, it says that the applied credit was only $40, the next payment will cost $0, and my remaining credit is $15.
If I had $40 initial credit and then was credited $20 through the referral, then shouldn't the applied credit be $60 and shouldn't the remaining credit be $35?
For context, I activated Monday and the next payment is 20th October.
The referral credit should show in your payment summary after the 20th. The same applies to any additional referrals you make - they will show after your next billing date.