This is a combo deal - JBHifi currently have 20% off iTunes cards as per here:
http://www.jbhifionline.com.au/ipod-mp3-portables/itunes.htm
(http://www.ozbargain.com.au/node/32811)
And they also have 10% PayPal cashback on orders over $50:
http://www.jbhifionline.com.au/paypal_cashback.htm
(http://www.ozbargain.com.au/node/32808)
So if you purchase at least a $50 iTunes cards via PayPal on JBHifi Online, you will get it for 28% off - ie $36 for the $50 card. Note: I haven't actually tried this year - and probably can't guarantee it will work until the PayPal cashback comes back in (if it does). The cashback only specifically excludes JBHifi gift cards, not itunes ones.
Note the comments below - probably need to buy $70 worth of cards to take advantage of this…
I think you'd have to buy at least $70 worth of vouchers, because the cashback is only $50 or above, so a $40 order for one $50 card would not satisfy the cashback requirements. Good idea otherwise.