The sign read "**** me Robbie" and was put on the big screen during the show. Williams made a play of it, asking "what do you mean?"
Hall left New Zealand four years ago when Williams toured for the first time in years, going to see his concert in Switzerland, afterwards living in London for two years.
"I followed him round for two years and took that sign everywhere.
"He recognised it when I held it up."
On the other side of the sign is the less confronting "New Zealand loves you".
"I always hold that up when we're overseas and he always gives me the thumbs up.
"He recognises me."
Saturday's concert was her 13th, but the first time she had touched the pop star.
After seeing the sign about nine songs into his set, he told the audience there were "no better offers tonight" and asked if she wanted to go on stage.
Williams was always the "classic gentleman", Hall said, and asked her partner Matt Challenger if he could borrow her, to which Challenger, also a big fan, agreed.
"I just shook," she said of her time on the stage.
"I just held him and shook."
The incident was not without its painful moments.
Williams had to tell Hall not to hurt his back — he has problems with back pain — and Hall had to tell him not to hold her hand too tight as she had recently broken her finger.
The pair sat down on stage and Williams sang a song, during which he kept whispering to her she was doing OK.