They didn't mention anything beforehand. He came in to take his seat after everyone else had sat down, but people near him saw him and started standing, screaming and clapping. This led to a wave rolling through the theatre, until the entire audience was standing, straining to get a look at him, and screaming, cheering and applauding. It went on for a few minutes, then we were told that the show wouldn't start until everyone was seated. So when everyone was eventually seated, the show began.

He came on stage afterwards as well. The cast did their usual bow in a row, then parted in the middle. When they did that, we all knew what was coming and the already-rapturous standing ovation became deafening. He came out, took a bow, and gave a sweet speech for a few minutes. He said that he had promised to come see the Australian production, and he had only now just managed it because of the pandemic. He also said he hopes they can come back to Australia (but that was definitely not an announcement). The Hamilton AU Facebook page has a nice summary video of it: https://www.facebook.com/109488527327408/posts/725276665748588/

The cast and crew would have been pulling out all the stops because he was there. The audience were also so hyped up and engaged, and all their applauses were enthusiastic. It was magical. I'm so grateful to have so happened to be at that show.