A NSW Government website

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.

Water quality report card

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.

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.

A

Algae

B

Water clarity

A

Overall grade

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).

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

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

Go to estuary report cards to find out what each grade means, read our sampling, data analysis and reporting protocols, and find out how we calculate these grades.

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

Historical water quality 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.

YearAlgaeWater clarityOverall grade
2010–11ACB
2013–14BAA
2014–15DBC
2015–16BAB
2016–17CAB
2017–18CBC
2018–19CCC
2019–20BBB
2020–21ABB
Physical data

Physical characteristics

Estuary typeBack-dune lagoon
Latitude–33.75 (ºS)
Longitude151.3 (ºE)
Catchment area4.3 km2
Estuary area 0.3 km2
Estuary volume12.7 ML
Average depth0.1 m

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.

Get involved

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.
A view of Dee Why Lagoon surrounded by the Sydney suburb of Dee Why and Dee Why Beach in the foreground

Aerial view of Dee Why Lagoon estuary

Local government management

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

Threatened species

Estuaries are important to our native animals as they provide food, shelter and breeding grounds.

Find out more about the biodiversity in our estuaries.