The flies were mainly on the underside of the flower and noticeably concentrated on the red parts, as seen in the second image. Most of the flies flew away once disturbed by the camera. Thanks to Marcus Leach for the initial sighting. Possibly the same type of fly as sighting 4375394 which was in the same area.
This looks very much like Scaptodrosophila hibisci, which has a natural distribution close to the coastline and north of Sydney- so perhaps another inveterbrate species translocated to the Botanic Gardens