There would not be much of a difference than any other cruise, even though you are leaving from Sydney there will be other nationals joining there as well as the Australians.
We did a similar trip last year. Generally the Aussies "dress down", but there were a fair proportion of Tuxedos and DJ's on board, but also a lot more suits or even Blazer and jackets than we had encountered on previous cruises. Most of the ladies wore long, be it a dressy summer dress or evening trousers. One thing we were aware of, and actually complained about, was that there was no effort to enforce the dress codes, which meant that people came in to dinner in what ever they wanted! The maitre d' said "he had no authority to enforce the dress code and that it was voluntary". What he really meant was that he didn't want to get into an arguement with a stroppy passenger!
Thankyou for your answer, we are Aussies and live in Melbourne as I stated we have found P & O ships to be a lot more formal, my husband always takes his DJ when we cruise from UK but wondered if just a suit would be okay since we thought there would be more Aussies on board. We complained on our recent cruise on Arcadia that the dress code was not always adhered to by passengers and complained but got a similar answer re enforcing the dresscode. It spoils the cruise when you make the effort to dress appropriately.
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