Phosphate levels should be less than 0.045mg/l for coastal plains (eg Maribyrnong at Keilor) or less than 0.025mg/l for uplands of the Maribyrnong (Sunbury and upstream). Excess phosphate (0.1mg/l or more) is of concern because it supports algal growth.
Measurements have been separated into sites upstream or downstream from Sunbury. (Note different scales in the top two graphs.) Phosphate levels are strikingly higher as the creek passes through Sunbury (see bar graph), possibly due to problems with old sewage pipes or to storm water runoff from the streets. Note that the Sunbury Sewage Treatment Plant occurs between the Macedon St Bridge and Holden, but the problem begins upstream of the plant. The phosphate is apparently diluted out by the time the stream reaches Organ Pipes National Park, except for one worryingly high result there. (To advance, click link below.)