Narrabeen Lagoon

Our water quality monitoring program has shown Narrabeen Lagoon to have good water quality. Find out more about the estuary and its unique features.

Narrabeen Lagoon is situated in the Sydney Metropolitan region of New South Wales. It is classed as a lake that has an intermittently closed entrance.

Northern Beaches Council open the entrance artificially when the water level reaches a specific trigger level, when flooding threatens surrounding infrastructure.

Estuary health and features

Water quality

As part of our water quality monitoring program we assess the water quality and ecosystem health of an estuary using a range of relevant indicators. We have supported Northern Beaches Council develop and carry out a monitoring program to assess the water quality of estuaries in their local government area. Sampling is carried out fortnightly between October and April each year. Three sites are sampled in Narrabeen Lagoon.

The monitoring program has been ongoing since 2012. The most recent sampling was completed over the 2021–22 summer.

Narrabeen Lagoon water quality report card for algae and water clarity showing colour-coded ratings (red, orange, yellow, light green and dark green, which represent very poor, poor, fair, good and excellent, respectively). Algae is rated 'good' and water clarity is rated 'excellent' giving an overall rating of 'good' or 'B'.

This report card represents 2 water quality indicators that we routinely measure: the amount of algae present and water clarity. Low levels of these 2 indicators equate with good water quality.

The report card shows the condition of the estuary was good with:

  • algae abundance graded good (B)
  • water clarity graded excellent (A)
  • overall estuary health graded good (B).

The past results of the monitoring program, including detailed water quality report cards, are available on the Northern Beaches Council website.

Find out more about our estuary report cards and what each grade means. Read our sampling, data analysis and reporting protocols and find out how we calculate these grades.

We have monitored water quality in Narrabeen Lagoon since 2010. This table shows the water quality grades for this estuary over that time.

Narabeen Lagoon historic water quality grades from 2010-11 for algae and water clarity. Colour-coded ratings (red, orange, yellow, light green and dark green represent very poor (E), poor (D), fair (C), good (B) and excellent (A), respectively).

Grades for algae, water clarity and overall are represented as:

  • A – excellent
  • B – good
  • C – fair
  • D – poor
  • E – very poor.

As part of our Beachwatch program we assess swimming suitability at 2 swimming sites in Narrabeen Lagoon using microbial indicators. We collect samples at each site weekly between October and April, and monthly from May to September, to calculate the suitability grade. Sampling is ongoing. Each grade reflects the most recent 100 water quality results to April 2022. Find out more about how we monitor beach water quality.

Local government area Swim site name Grade
Northern Beaches Narrabeen Lagoon (Birdwood Park) Poor
Bilarong Reserve Poor

Estuarine, lake and lagoon water can sometimes be unsuitable for swimming, especially after rainfall when stormwater and wastewater can overflow into swimming areas. Learn more about what the grades mean and how we grade swimming sites on our beach classification webpage.

Physical characteristics

Estuary type: Lake

Entrance
location 		Latitude (ºS) –33.7
Longitude (ºE) 151.31
Catchment area (km2) 52.4
Estuary area (km2) 2.3
Estuary volume (ML) 5251.6
Average depth (m) 2.3

Tidal exchange volume

Tidal exchange volume or tidal prism data is available for this estuary. This tidal prism was measured in 1980.

Tide state Flow
(106 m3)		 Local tidal
range (m)		 Sydney Harbour
tidal range (m)
Ebb flow 0.28 0.14 1.08
Flood flow 0.5 0.19 1.22
Notes: km2 = square kilometres; m = metres; m3 = cubic metres; ML = megalitres.

Water depth and survey data

Bathymetric and coastal topography data for this estuary are available in our data portal.

Land use

Narrabeen Lagoon drains a mostly forested catchment, part of which is protected in a conservation area. Urban development is the other main land use within this catchment, and occupies about one quarter of the total catchment area.

National and marine parks

  • A large section of Garigal National Park is located within this catchment.
  • This estuary does not flow into a marine park.

Community involvement

  • The Coastal Environment Centre in Narrabeen, run by Northern Beaches Council, educates the community and provides opportunities for locals to get involved in coastal environmental management.
  • Northern Beaches Council also facilitate a Bushcare Program for local residents to get involved in conservation and restoration of the natural environment around Narrabeen Lagoon estuary.
Narrabeen Lagoon

Local government management

Local councils manage estuaries within their area unless the estuary is attached to a marine park. Northern Beaches Council manage this estuary.