Fishing at Ginninderra Spillway is as such fine and leagal, but there are two major issues with it where some people behave wrong and lead to negative externalities for others.
1. leaving the fish caucus (typically carp) to rot nearby. (Externality: it stinks, dogs roll in it and attracts vermin)
2. people illigally drive all the way to the spill way, on the way they cross two green strips and a bike path, as well as two rows of destroyed bollards. (Externality: danger to bikers (collision and lots of dirt on bike path where they cross it), destruction of nature and extra erosion along illeagal rout)
So please, when you come fishing here, behave properly: Walk here rather than drive and dispose of any dead fish into the garbage.
If you see others misbehaving while you are here, feel free to call 132281 (option 1 then 5 then 1) and report them to the Rangers. And you can always give them the evil eye or voice your concern if you feel like it.
Good luck with your hunt, consider coming back if it is busy at GZ (this would be rare).