I flew Melbourne to Hobart (and back) at 31 weeks, no problems at all.

I did get a letter from my doctor saying I was OK to fly, but nobody asked to see it.

I believe that most airlines will take you routinely up until 36 weeks.

Make sure you drink lots, as the air-conditioning means you might get more dehydrated than usual, and get an aisle seat so you can stretch your legs if you need to. You'll be fine.