I flew to Sydney and back (from Adelaide) at 16 weeks, and Perth and back at 21 weeks. Somewhat uncomfortable, but nothing major. I'd say that your health professional recommended not flying OS in the first trimester due to the risk of miscarriage.

What I mean by that is that if you are going to miscarry, it'd be better to do it on the ground, in your own country, with access to medical help, than in mid air, or in an unknown country, with a potentially inadequate health system.