The only eastern Melaleuca even possible is Melaleuca squarrosa. I do not think it is that. Does anyone have access to planting records for this site? That may give a clue.
Hi Michael M. I think you are right and have confirmed your suggestion. I have added another photo which shows the fruit at different stages of maturity. See: http://canberranaturemap.org/Community/Sightings/Details/1974298
Also, an interesting quote: "It is an adaptable shrub, grown in many parts of Australia for its attractive foliage but is regarded as an environmental weed in parts of Victoria." From: https://en.wikipedia.org/wiki/Melaleuca_decussata