Yes, you can buy a greenslip for 6 or 12 months. However, you have to register your vehicle for 12 months if:
This is the first registration of a vehicle in NSW.
Registration (6 or 12 months) has lapsed by more than 21 days.
Your vehicle needs a blue slip because it was transferred from interstate or has a cancelled registration.
After the first registration, you can buy a greenslip and register for 6 months.
greenslips.com.au provides the cheapest CTP greenslip prices for 6 or 12 months.