I'd think you'd be fine.
The reason for this from a personal experience was that I was suffering greatly on the progesterone phase of my first lot of HRT with BAAAAD Pmt symptoms (It was synthetic prog) . It was absolutely awful.
I went back to my GP and she happily gave me 3 month cycle HRT where you just take the progesterone phase once every 3 months.
Unfortunately I was awful on THAT progesterone too as it was also synthetic. I awaited an appointment at the gynae clinic and I asked if I could forego the prog until I saw the specialist and I was told yes.
So it seems to be OK to miss for a period of time, for me - 3 months, so I should imagine skipping for one month would be fine.
Our Hurdity on here knows an awful lot about progesterone and Utrogestan and I'm sure she will be willing to give her opinion if she sees this thread.
Have a brilliant time in Australia!! I'm dead jealous!!!