Sorry gents.
Clean forgot about this.
The earliest record I'd found was a 1928 semi final between St.george and Souths (working from memory here as it was in a book at home)
But if indeed an extra time game was played in 1920 then so be it, although i'd be interested to know why, considering there were no finals and Balmain were miles in front of every other side and thus, declared Premiers without any finals matches played, extra time seems like a silly notion.
However, I'm not going to state it's wrong, as I've not researched that yet, so We'll go with correct.
Someone else can have a crack