I haven't been to Sandy Point for over 5 years, but back then I thought a volunteer group with roundup could have halted the spread. I've been in the weed control game for the 5 years since then, sprayed the odd bit of Marram grass we get in the Wollongong local dunes, and I'm now sure it could have been controlled. You'll have to get Parks on side, get chemical certificate qualifications, (not hard) and learn to identify Marram Grass. Don't poison the native grasses!
Start with the small pioneering tussocks, see how many the team can treat in a few hours, mark them (or the area) up with your gps, and go back every couple of weeks until they're dead. Don't expect Marram grass to die first hit. Most things with tough leaves take a few goes. With the good areas under control go to work on the edges of the thicker stuff.
The hard part is organising approval with the authorities and then finding the volunteers prepared to spend a couple of hours every couple of weeks. A quad bike would have been handy when I last saw it. But if the tussocks really have closed up maybe not now.
www.iucngisd.org/gisd/