You can start your registration requirements and gather evidence before you have completed 80 days of teaching. However, you will need to have completed at least 80 days of teaching before you apply for full registration.
Remember, the 80 days is a minimum requirement and is designed to be a time where you move your practice to the proficient teacher level of the Australian Professional Standards for Teachers (APST).
For teachers returning from non-practicing, the requirements depend on how long it has been since you have maintained proficiency through the annual renewal process.