Agree it's a PITA, but all sites do it at some point. The alternative is to try and update without removing, which usually results in all sorts of stuff-ups. I've been locked out of QF when I've been making travel plans late at night (QF have only very recently set up a dummy site to take bookings during this time, though it rarely works). I've also been locked out of AA, BA, U2, National Rail and Amtrak sites around the same time (for the local company HQ). Companies generally do it the time they receive the least amount of traffic. I can't imagine an Oz domestic carrier having a peak period during late Sunday night Oz time.
It seems as though it was only down for a small amount of time anyway (was working 20mins after OP posted). I agree a timeframe would have been nice though, the "shortly" part of "we will back shortly" doesn't really tell you anything.