I attempted this last night as well with no credit appearing upon checkout. I did an online chat with the online customer service and apparently the promotional certificate only applies to items sold by Amazon AU.

So even though you are purchasing the item from the Australian Amazon website, the seller details will show when you select an item. If the seller is anyone other than Amazon AU then the credit won't be applied (my item was being sold by Amazon US, even though I was on Amazon.com.au).

I hope that helps.