Airways seem to compete for lowest fares. Jetstar offered a similar sale from Melbourne to Darwin.
Here's from Tiger Airways. Travel between 1 Dec 07 - 28 Mar 08:
from A$79.99 (one way) Melbourne-Darwin (Includes taxes and fee)
from A$499.00 (return) Melbourne-Singapore (Flight Combo) (Excludes taxes and fee)
from A$119.99 (one way) Melbourne-Singapore (Flight Combo) (Excludes taxes and fee)
Not sure how long will this offer stay since it's not mentioned on the website.