The speed tier you need will depend on your connection needs and those of the people living with you (if any). The following speed tiers are available through the NBN, with each representing the maximum possible speeds:
NBN 12: 12Mbps download/1Mbps upload
NBN 25: 25Mbps/10Mbps
NBN 50: 50Mbps/20Mbps
NBN 100: 100Mbps/20Mbps or 100Mbps/40Mbps
NBN 250: 250Mbps/25Mbps
NBN 1000: 1Gbps/50Mbps