The monument mall officially opened in 1992 (some sources say 1990 but that could have been when construction started), the top floor where the Botanist is was originally the food court, the mall wasn't really as successful as hoped and didn't attract the footfall it hoped. It's changed hands a couple of times and is mainly food and drink offerings on the west side, high end retail on the south and the Northumberland Street side is an entrance to a large tk max store There's rarely an empty unit nowadays so the blend seems to be right.
Previously there was the Post Office and I think either a Burtons or Jackson the tailor on the original footprint.