The fish gate allows fresh sea water through the Wonnerup floodgates to boost oxygen levels in the estuary and provides an escape route for fish.

Department of Environment and Conservation (DEC) scientist Jim Lane checked the electronic fish gate, midway through last week, and found it to be functioning correctly.

However, the gate closed on Saturday, and with the high levels of algal bloom in the estuary, oxygen levels quickly depleted and about 600 fish, predominately black bream, were reported dead on Sunday.

On Monday morning the scale of the disaster became apparent when DEC and Department of Water staff started cleaning up after the first major fish kill in the area for a decade.

The gates, controls and monitoring equipment are Water Corporation structures, operated under delegation by DEC during the summer and autumn months.

Water Corporation’s regional business manager Scott Moorhead said the gate was operated remotely by DEC via a system that communicates with the corporation’s operation system.

The gate’s position can be monitored from the corporation’s Busselton depot and an alarm is triggered if the gate fails.

Over the weekend there was a problem with the corporation’s remote communication system and the alarm was not detected.

“The corporation is currently working to resolve these communication issues and in the meantime the gate will be operated and monitored manually by DEC,” Mr Moorhead said.

The Department of Water has overall responsibility for investigating the fish kill and its South West regional manager Wayne Tingey said the focus was on the malfunctioning gate and recent levels of toxic algae.