Well going by your speedtest report, you have a maximum upload speed of just under 2000kb/s. Unfortunately there is not much you can do, you may be lucky to push out a bitrate of 1k, as you have to also account for the bitrate of your audio. You could lower the audio to 64 (Minimum recommended by Twitch) which would allow a slightly higher throughput of video bitrate, I wouldn't go past 1250 bitrate though as you also have background apps, system, etc. using bandwidth every so often which may interrupt streaming throughput.
Slightly off topic:
What I would personally do is contact Telstra tech support and try to get them to remove whatever cap they have in place for your area in terms of upload speed, because it is really unsatisfactory in this day age. If you can do a download speed of ~100Mb/s then you can do the same for upload, as long as your PC and modem can handle the throughput at such speeds.
I'm on ~40Mb/s down, ~20Mb/s up so you should have far better upload than that. Hopefully you can get a tech support agent that is more than just a sales support agent (So many of them at Telstra) Tell them that you are doing streaming which requires more bandwidth in terms of upload than you are getting with your current plan, don't let them hoodwink you into doing a new plan for a better deal, as you are already subscribing to their Speed-boost service which IMHO is a joke as it proves they are shaping their customers bandwidth.