Four lagoons in the line

Beachwatch's report names lagoon discharge as a potential source of contamination at five of the twenty: North Narrabeen Beach from Narrabeen Lagoon, Long Reef Beach from Dee Why Lagoon, North Curl Curl Beach from Curl Curl Lagoon, and Queenscliff and North Steyne beaches from Manly Lagoon.

The four, side by side by average depth

The NSW environment department (DCCEEW) gives each of the four the same set of figures: estuary type, area, volume and average depth.

The four lagoons, from the NSW environment department (DCCEEW): estuary type, average depth, estuary area, catchment and the year its water quality monitoring began.

Lagoon	Type	Average depth	Estuary area	Catchment	Monitored since
Narrabeen Lagoon	Lake	2.3 m	2.3 km²	52.4 km²	2010
Dee Why Lagoon	Back-dune lagoon	0.1 m	0.3 km²	4.3 km²	2010
Curl Curl Lagoon	Lagoon	0.3 m	0.1 km²	4.7 km²	2015
Manly Lagoon	Creek	0.4 m	0.1 km²	17.2 km²	2007

By the department's figures, Narrabeen Lagoon averages 2.3 m; Dee Why averages 0.1 m, Curl Curl 0.3 m and Manly 0.4 m, all three of them under half a metre on average.

Narrabeen Lagoon covers 2.3 km², Dee Why 0.3 km², and Curl Curl and Manly 0.1 km² each, but the volumes fall in a different order: 5,251.6 ML at Narrabeen, then 35.6 ML at Manly, 20.5 ML at Curl Curl and 12.7 ML at Dee Why, the second largest of the four in area and the smallest in volume.

Narrabeen Lagoon drains 52.4 km² and Manly Lagoon 17.2 km², against 4.7 km² at Curl Curl and 4.3 km² at Dee Why.

What the land is used for differs as sharply: Narrabeen's catchment is mostly forested, part of it protected in a conservation area, with urban development about one quarter of the total; Dee Why's is highly disturbed, with almost the entire catchment used for urban development; virtually all of Curl Curl's is urbanised, some of it for commercial and industrial purposes; and Manly's is highly disturbed, with about one quarter of the land still forested.

What lives in and around them differs too. Narrabeen Lagoon is important habitat for threatened species including the red-crowned toadlet and the spotted-tail quoll, and a large section of Garigal National Park sits within its catchment; the Dee Why Lagoon catchment has no conservation area at all. Manly Dam, built in 1892, regulates the major creek that feeds Manly Lagoon.

The department monitors water quality in all four, sampling fortnightly between October and April each year, and on its current report cards Narrabeen Lagoon, Dee Why Lagoon and Curl Curl Lagoon have fair water quality and Manly Lagoon poor. At Narrabeen and Manly the most recent sampling was completed over the 2023 to 2024 summer, and the department's own page carries the current grade.

The biggest and deepest is Narrabeen Lagoon, with a mostly forested catchment behind it. The shallowest is Dee Why Lagoon, a back dune lagoon whose catchment is almost entirely urban. Curl Curl and Manly cover the least area, 0.1 km² each, and Curl Curl is the most recent of the four to join the monitoring program, in 2015. Manly is the one classed as a creek, holds the longest monitoring record of the four, from 2007, and carries the poorest water quality finding of the four.

After rain, and when an entrance is open

Beachwatch's advice for this coast covers the water behind the beach as well as the surf in front of it. In its own words: "As a general precaution swimming should be avoided during and for at least one day after heavy rain at ocean beaches, and for up to 3 days in estuarine and lagoon areas, or if there are signs of stormwater pollution such as discoloured water or floating debris."

Where a lagoon entrance is open, the advice is more specific still: "Swimming is not recommended at ocean beaches located near lagoon entrances if the lagoon is open, due to the possibility of pollution from the outflow."

Narrabeen, Dee Why and Curl Curl each have a water level set as a trigger, and when the water reaches it the entrance is opened artificially; for Narrabeen the department gives the reason, that flooding threatens surrounding infrastructure. All four are described at the entrance the same way, intermittently open and closed to the ocean, which is the condition Beachwatch's advice is written around.

Beachwatch also keeps two sampling sites inside Narrabeen Lagoon: Birdwood Park, a sandy beach on the southern side of the entrance, monitored since 2004, where the entrance has been periodically open and closed at times, and Bilarong Reserve on the northern shoreline of the lagoon, monitored since 2014.

More of this coast: Pittwater, the other shore, and the twenty ocean beaches, one by one.

The whole coast, Palm Beach to Shelly Beach