Very good question davidp, on a topic I checked into myself recently. In NSW, according to the Office of Fair Trading, the by-law becomes effective as soon as it is appropriately approved at a General Meeting. Ludicrously though, the Owners Corporation's Executive Committee and / or Strata Manager have two years (yep, two years!) to actually register it.
What you often find is by-laws get approved then forgotten about and never registered, in which case they 'expire' after two years. Everyone might think a by-law exists because it was made yonks ago, but in reality it doesn't because the slackos never registered it.
Or in a Joseph Heller-esque scenario, a new by-law gets voted in but nobody knows because the Executive Committee and / or the Strata Manager don't spread the word.