For those that use North American PSN accounts. Be sure to double check the correct country you intend to buy. Lastly, PCGameSupply may require SMS verification for security purposes.
CA CAD $20 PSN Card = AUD $17.81 (Requires Canadian PSN Account)
US USD $20 PSN Card = AUD $23.33 (Requires United States PSN Account)
Update 0: Thanks to Nismo for confirming that the deal is only viewable as a Desktop.
Update 1: Thanks to Innamura, for pointing out that the offer is valid for only one card per account.
"SMS verification" = they sell your number to spammers, so use a burner number unless you don't mind frequent international calls from UK and Canada numbers