The NCA now has a website (http://www.nationalcapital.gov.au/waterquality) where people can check LBG water quality, alerts and closures. This site may have been around for a while but I have just found it.
The data looks OK, although I often walk around East Basin which supposedly has very low algae concentrations (only 20 cells/mL, compared with Black Mountain Beach’s 8610), and East basin looks to me to be highly infested with algae.
Anyway, worth a look.