To be honest, that modem is rubbish. Buy a better modem, you will most likely either get a better sync or better speeds, or both. I upgraded from that modem 4 years ago to a Billion 7800N (really good at the time, old now) and my sync speed improved and so did my speeds, by double infact. I went from 1MB/s speed to 1.5MB/s speeds just by upgrading modem. When I played around with some settings in the new modem which that modem you have doesn't have, I increased it again to 2MB/s. Unfortunately I moved states and those speeds are long gone.
But, if you don't want to buy a new modem that is cool, though try this. In your modem settings, change your DNS IP Address allocation to manual, and enter the IP addresses on the Telstra website for the state you live in. In the last few weeks, whenever I have used obtain DNS IP Addresses automtically, my speeds are 100KB/s, when I change to my state's DNS IP addresses, my speeds are 1.3MB/s. Another person (in a different state to me) on this forum followed my advice and received speed increase from 100KB/s to 500KB/s. There is something horribly wrong with Telstra default DNS IP addresses at the moment.