What happens when a player wants to leave or join the team mid-season?

Leaving: an Amateur cannot be transferred within Australia or loaned for any consideration, such as a Transfer Fee or Loan Fee. An Amateur who no longer wants to play may let the registration lapse. To change Clubs during a Season, the Amateur must sign Prescribed Form NRSTR08 (Cancellation of Amateur Registration) and send a copy to the Club and the Competition Administrator, or apply to de-register in the National Online Registration System. De-registration takes effect immediately once the Club confirms the notification, or seven days after the Player requested it, unless a Grievance has been started with the Competition Administrator.

Club ending a registration: if the Club no longer wants an Amateur to play, it must use NRSTR08 or apply to de-register in the National Online Registration System, which notifies the Player and the Competition Administrator.

Joining: a Player who was previously registered with a Club cannot be approved in the National Online Registration System until a National Transfer has been created in, and captured by, the National DTMS, and the Club must not let them play in a Match until it has received a valid Domestic Transfer Certificate. Registration Periods do not apply to Amateur Competitions: the Competition Administrator sets in the Competition Rules the timeframes within which Players may be registered.

Limits: a Player may be registered with a maximum of three Clubs in each form of the game in a Season, and is eligible to play Matches (other than friendly or trial matches) for only two of them.

Source: NRSTRs, articles 6.6, 6.7, 6.10(e), 6.13(d) and 12.