GHCMA executive manager of waterways Chris Solum said the estuary had been closed to the ocean by a narrow bank of sand and there had been several illegal attempts in the past week to open the river to the ocean.

"We have been actively monitoring the estuary. Low seasonal rainfall during January to May led to low freshwater inflows and very low dissolved oxygen levels in the estuary. This has meant there has been no safe or appropriate opportunity to artificially open the estuary this year," Mr Solum said.

The estuary started to open on the morning of Sunday June 26, and drained rapidly from 11am onwards.

When the river opened, the oxygen-rich water at the surface flowed out to sea, leaving anoxic water behind. Without any oxygenated water the fish and other animals in the river such as tupong, bream, gobies, smelt, mullet, gudgeons and shrimp, died.

Mr Solum said the major kill was "extremely unfortunate" and suggested the premature opening wasn't entirely natural.

"We, and a number of community members, have been particularly disappointed to have had to intervene to prevent several illegal artificial opening attempts in the last week," Mr Solum said.

"It appears the opening that occurred on Sunday happened where the integrity of the sand berm had been previously compromised by one of these attempts."

Mr Solum said the GHCMA was the responsible authority for opening river mouths in the region and would only do so under specific circumstances and after rigorous analysis.

"I would ask those who have tampered with the estuary mouth to stop taking matters into your own hands," he said.

It is suspected those who tried to open the river were concerned about high water levels in the estuary, but Mr Solum said that was no excuse for their illegal actions.

"These seasonal closures and the temporary high-water levels they create are extremely important for the estuary's ecology, including fish, birds and native vegetation," he said.

Investigations regarding the opening attempts are ongoing and monitoring of the estuary will continue in the days ahead.

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