When has any AFL or VFL team goen undefeated in a season?

This has never been achieved. Although, in 1929 Collingwood went through the full season undefeated. Collingwood lost to Richmond in the first semi-final but went on to win the Premiership. Under the then rules, the minor premiers were always in the Grand Final. So the answer is that it has never happened that a team has remained undefeated for a whole year.