CTP Greenslips are included in your ACT rego. Compulsory Third Party (CTP) is insurance for third party INJURY ONLY, not to be confused with damage to vehicles, thats why you always hear people being pinged for being both "unregistered and uninsured". They don't mean the person wasn't insured for damage, because that type of insurance is not compulsory - I often see people get confused in this area.
Many years ago they deregulated greenslips in NSW and therefore made it more competive and brought registration costs down. One example is the F250, ACT rego is over $3,000, in NSW its $700 (inc greenslip).
Those who register in ACT won't need to worry, its already fixed price and included in your ACT Rego and as Frink said, appears to be underwritten by NRMA.