So, looking at the Vic NPL PPS regs from last year, there doesn't seem to be any special considerations for such situations, so I'm assuming it will be the same as other NPL clubs (at least for within the NPL season). In the Y-League, it looks like something different.

NPL allows 2 foreign players in the squad, and a maximum of 2 players on match-day.
Y-League seems to state that any foreign players that are signed as part of the first-team, AND are under-21, will be considered an overage player during Y-League matches (which is a maximum of 4)