I believe it is determined by total head-to-head points against. For example, if Team A beat Team B by 50 points in the first match-up, but Team B beat Team A by 60 points in the second match-up, then the tiebreaker would go to Team B as the total points scored against was ten points higher IN TOTAL for Team B.
This is how we've been doing it for the last three seasons.
Sorry to be a stick-in-the-mud, but this way of calculating has cost me in the past and I would side with keeping it the same at least for this season.
We can always vote for a change starting next season, if everyone so chooses.
If three teams tie, then I would be in favour of just going with highest overall point total, as was decided a couple of weeks ago in a debate.
My (humble) opinion.