It generally closes at points in the day but the opening bands are reasonably broad and I believe (but cannot 100% confirm) that they have the contract for SQ lounge access which might account for the slightly extended hours

Noting that it’s an ancient thread will add a couple of general points
-it definitely doesn’t open specially for JQ flights or a lot of QF codeshares eg on TL
-the aero club is long gone but the Novotel was selling day passes for the rather flash pool which could be a decent alternative

Darwin Airport can be very hit and miss of late on the time to get through security. You can find yourself confronted with some insane queues so while my previous advice would have been to leave it fairly late before heading up, these days I probably wouldn’t rely on that

if I am passing before your flight I will try pop in and pap the current opening hours