If the reduction in speed is fairly sudden, then go through the standard checks … isolation test, changed configuration, any new software installed (including viruses), etc. Note also that ADSL2/2+ speeds can vary a fair bit from day to day.
Assuming that the reduction is gradual over an extended period; it is likely caused by cross-talk as more users on the same cable run as you sign up to ADSL2+ services.
To explain … any signal travelling along a wire will induce a magnetic field around it, which can induce an electrical current in neighbouring wires – known as crosstalk. As more of your neighbours adopt ADSL2+, the result is an increase in crosstalk noise … therefore a decrease in SNR which will cause your sync speed to suffer. Naturally, the more phone lines with ADSL2+ on them in a bundle, the more crosstalk will be present and the signal quality for each ADSL line will be worse.
As more users connect, your speed will continue to decrease and those on the extremities in some instances will lose sync. Unfortunately there’s not a huge amount you can do about this, other than ask Santa or your local politicians to change Telstra’s policy about Fibre to the home.