According to the Laws of Australian Football the offending team loses their whole score, gives away a free kick and a 50-metre penalty. You may also have read there was a similar incident in the NEAFL Grand Final on the weekend where the 19th man for Southport was identified by Sydney's captain while still on the field. In that case, a decision was required in accordance with the applicable NEAFL by-law
"Where a player does not enter the playing field in accordance with these Rules, the Match Manager and/or field umpire(s) must report the circumstances to the AFL. Time and scores at the time of the breach are to be recorded. The AFL may determine the matter by way of fine, reversal of Match result or other sanction as it deems appropriate [my emphasis]."
I'm unsure what SANFL's by-laws say about the issue, but it could have been that North's score may not have been wiped automatically had they been 'caught in the act', so to speak.