Hey Barkath,
We have posted a guide as to some basic troubleshooting you can try here: https://crowdsupport.telstra.com.au/t5/Home-Broadband/Top-ADSL-troubleshooting-tips/td-p/158048 and a guide specifically related to speeds here: https://crowdsupport.telstra.com.au/t5/Home-Broadband-KB/How-To-Troubleshoot-Speed-Issues/ta-p/278139
Can you please try both these and report back?