I'm currently on a plan with Optus for unlimited data on ADLS2+ and am getting max speeds, while connected with Ethernet mind you, of 250kb/s.
This is gross.
Optus doesn't have cable to my place and NBN isn't being rolled out to my location until early to mid 2020.
Is there anything I can possibly do, otherwise I'm cancelling my contract and going with Telstra as they have cable to my house.
Cable is almost always going to be superior to DSL so if you have the option and speed is really important to you, I'd switch.
What's your line sync speed though? If it's close to the real world speeds you're achieving, it's likely that you're too far from the exchange to get better speeds then this.
Unfortunately there is no minimum speed technically for ADSL. I think your speeds translate to around 2Mbps. ADSL2+ should get 1-20 Mbps. Cable gets 25Mbps or 100Mbps depending on what plan your choose.
Unfortunately its unlikely Optus can provide speeds any faster than what you are getting. I would recommend Optus 4G mobile broadband (if available in your area) at 10Mbps but if you have access to cable that is far better again.
Loath to recommend a competitor but if its a choice between ADSL and Cable then I would recommend the cable every time. An added benefit would be the NBN are using the Telstra cable network so if they go HFC in your area then you already have a lead in installed.