The Fisheries and the Department of Lands - two of the LEAST 'Green' and most bureaucratic government departments in my experience - will not approve ANY new development that threatens weed beds, particularly Posidonia strap weed or Zostera eel-grass. This rule came in in the early 2000s, from memory, and is a blanket ban - even when the dredge the Port Hacking, the dredge is meant to stick to the main channels where this weed doesn't grow, and avoid disturbing it where they can (obviously more of a guideline in this case).
More of an issue at the Kurnell boat ramp is the fact that it abuts a groyne - any dredging that takes place there will be filled in again with sand relatively quickly which is captured by the groyne. You can see how the sand is creeping up the concrete ramp. Unfortunately it's not worth the Council's time or money to get a dredge in every six months. The smart money would have been to replace the groyne with a longer, built up ramp to take boats past the weed and into deeper water, but that means big initial outlay for the Council, and they'd probably prefer to spend it on more cockatoo proof bins, or redoing the pavement in Sutherland than spend it on improving access to the water.