$20 Warehouse Mobile Top up for $10

Instead of offering 15% off everything at The Warehouse like at easter the Warehouse Mobile are offering customers a $20 top-up for $10. From the text:

HALF PRICE TOP UP! Buy a Warehouse Mob $20 top up for just $10, IN STORE ONLY, ends Mon 6th June. Show text at till to buy [Code:Fast Key92]. Limit 1 per customer.

Just quote the code to the staff or show them the text and you should be good if you haven't got a text. Similar to 2 Degrees offer. If you have received a similar text could you confirm that the code isn't unique.