JB Hi-Fi have 15% off iTunes gift cards at the moment online and instore. As usual it doesn't include the $20 gift card - $30, $50 & $100 gift cards only.
$30 gift card = $25.50
$50 gift card = $32.50
$100 gift card = $85.00
Should run for at least a week.
15% off @ Dick Smith as well
http://www.dicksmith.co.nz/15-pct-off-itunes-gift-cards