My Optus ADSL broadband speeds have reliably been around 4.2 Mbsp for over a year, and then around a week ago they have inexplicably dropped so now we're lucky to get 1.6 Mbsp even during non-peak times. We haven't changed any of our setup and am at a loss to even know where to start. Any suggestions for fixes?
Probably line quality based. Could also be corrosion in some contacts up in your house. Try measuring the peak speeds during midnight or early morning times. Turn the modem on and off. Try to see if each line you get is a different quality.
Have you had any rain recently? Sydney has been in drought.