JQ has different check in queues for each flight. At minus 30 the queue co-ordinator (dunno the exact title but they are out there directing the pax to each queue) ropes off the queue for that flight.
So, if you turn up 45 mins before and wait for 15 mins in the queue while a slow or new agent works through the line, it's tough cookies if you miss out???
That's an interesting concept ditzyboy. As apacau points out, if you had passengers unlucky enough that their queue was slower than the others for whatever reason (and there could be many), and they miss out by a couple of minutes, I'd be calling security!
Reading Ditzys explanation I interpret that as meaning when the 30min cutoff arrives, the queue is roped off and no additional punters can join it, the punters already there were there 30min before.
I still don't get what the big hassle is to turn up on time. unless it's something like the cabbies blocking the airport again.