The post above about the narcan injection is not true at all. I've heard rumours of that sort of thing happening in the US, but it's not the case here. To get on methadone, you need to go to a methadone clinic and go through an assessment process. This involves talking with either a social worker / psych / nurse about your drug use history / life circumstances. Then you'll see a doctor who'll usually ask for a urine drug test, as well as a general physical check-up.
I notice you're in Brisbane. You can call ADIS (Alcohol and Drug Information Service) on 3236 2414 and ask for the contact details of government methadone providers in your area. Some community health centres have methadone clinics attached.
A lot of people try Biala out first, because they're centrally located on Roma St. You could give them a call on 3238 4080. There's also the Peel St Methadone Clinic at South Brisbane. Their phone number is 3844 9222.
Also, some private GPs prescribe methadone. You can find one by calling the opiate treatments hotline on 3896 3805, but be aware that it can be really difficult to find a place with a private GP.
Both private GPs and the government clinics have a limit on the number of patients they can take on at any one time, so it might take a while. Drop me a PM if you'd like any more specific info.