Ahh TRUNKS, sucked in by the unfeasable (atleast in Aus, for the moment) unlimited DSL. All the leechers on one seriously underprovisioned network. Dodo, along with all (most) of the other unlimited DSL providers are on this networking, reselling the same service.
The slow speed is due to the fact that the contention ratio is 30/1 or greater. This means that for every 30 512k connections, there is only one real 512k amount of bandwith. Mmm 512k between 30 users, nice and slow.
Usually contention ratios that high can be workable, however unlimited does not attract those mum and dad type users who don't download too much, it is more likely to attract leechers, hence why 30/1 is unworkable, and you get shocking download speeds.
This is why you cannot do much to improve your download speed.