When you top up, service credits from that top up are transferred to your account balance. Those service credits are what's used to purchase your selected base plan and boosters. By regularly topping up your account, you'll ensure you have sufficient service credits to renew your base plan and ensure uninterrupted service.
Remember, you can always check the amount of service credits on your account at anytime by logging into Prepaid Self Serve or dialling *611 on your phone (it's a free call!).