As is usual when dealing with Webjet, ensure you know the fees+charges they tack on:
Here is a sample of fees from a quote for a Qantas Australia-New Zealand return flight:
Qantas Credit Card Surcharge $ 15.40 [not sure if this is charged when paying by Paypal ... ]
Webjet Booking Price Guarantee $ 12.95
Webjet Processing Fee $ 22.95
So, you're up for a min of $35.90 in additional fees to book through Webjet. Plus they also have hefty change fees if you don't change online (although they have a fairly good change system that you can use online_
That said, 20% off is pretty good... as long as you know what you're doing.