Dee Why Lagoon

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

Dee Why Lagoon is situated in the Sydney Metropolitan region of New South Wales. It is classed as a small back-dune lagoon with an intermittently closed entrance.

Northern Beaches Council has determined a water level height which, when reached, triggers the artificial opening of Dee Why Lagoon.

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 during the warm summer period. Two sites are sampled in Dee Why Lagoon.

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

Dee Why Lagoon 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 'excellent' and water clarity is rated 'good' giving an overall rating of 'Excellent' or 'A'.

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 excellent with:

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

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 the Dee Why Lagoon since 2010. This table shows the water quality grades for this estuary over that time.

Deewhy 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.

Physical characteristics

Estuary type: Back-dune lagoon

Entrance
location 		Latitude (ºS) –33.75
Longitude (ºE) 151.3
Catchment area (km2) 4.3
Estuary area (km2) 0.3
Estuary volume (ML) 12.7
Average depth (m) 0.1
Notes: km2 = square kilometres; m = metres; ML = megalitres.

Water depth and survey data

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

Land use

The catchment of Dee Why Lagoon is highly disturbed. Almost the entire catchment is used for urban development.

National and marine parks

  • There are no conservation areas within this catchment.
  • This estuary does not flow into a marine park.

Community involvement

  • Northern Beaches Council facilitate a Bushcare Program for local residents to get involved in conservation and restoration of the natural environment around Dee Why Lagoon.
Dee Why 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.