Large wasps, males about 30mm body length, females smaller; a gathering concentrated in several willow trees beside the Murrumbidgee River, to the north of Tharwa.
Sorry, the abdominal markings are quite wrong for Thynnus. The wings are also not hayline. There's an image of T. zonatus here: https://nre.tas.gov.au/biosecurity-tasmania/plant-biosecurity/pests-and-diseases/european-and-english-wasps