The relevant section of the memo from NZF about the changes...
No club can play any player who is not registered by this point for either the conference or championship phase.
No loan agreements or transfer periods will be in place as these terms relate specifically to professional players.
Players may only be registered for three clubs and play for a maximum of two of these within a season as per the NZF Regulations on the Status of Transfer of Players.
It's important to think of the 'national league' as of 2021 as a competition that starts in March (or April) with the conference phase and somewhere between 30 and 39 teams spread across three conferences and culminates with a championship phase thats starts in September/October and consists of 10 teams.
It's all one thing, it's not 'winter leagues' and then the 'best teams from the winter leagues playing in a summer league'.