Works out to half price for data. The 8/16Gb is to do with their off peak (nightime) is twice the data per unit cost.
Other specials too, all about "half price" (Dodo price?).
$10 for 3/6Gb mobile data, 30 day expiry.
$25 for a USB Mobile Broadband Modem with 5/10Gb data, 30 day expiry.
$50 for a Portable Wifi Hotspot (mifi pebble) with 15/30Gb data, 90 day expiry.
Deal ends midnight tomorrow (according to their web page)
Seems like they want to sell new stuff. For example I'm a dormant mobile broadband customer, yet there seemed to be no way to use my existing sim with this deal, only recharge it at the standard rate (twice the price!). So I ordered a new sim through the online form. I suppose it will arrive at my door in the next few days.
Note that the $10 deal is for a micro SIM, not a standard SIM.