Some of the nurdles – at least a million – were discharged from the treatment plant into the ocean, prompting an ongoing clean-up on beaches from Warrnambool to Yambuk involving multiple government agencies and up to 600 volunteers, including school children.

The Environment Protection Authority (EPA) has been investigating whether Wannon Water breached its licence to release treated water into the ocean – an offence – and Wannon Water has been investigating the source of the nurdles.

Those responsible for putting the nurdles into the system could face prosecution under the Water Act.

EPA south west manager Carolyn Francis said Wannon Water had to bear the costs of the clean-up in line with a notice it was issued requiring the clean-up of the incident and remedial action to prevent a recurrence.

Wannon Water acting managing director Steve Waterhouse said it had already spent $200,000 on the clean-up over the past six weeks, including employee time, labour hire and equipment.

He said Wannon Water had a dedicated incident team ensuring there was specific attention on the clean-up effort.

“We are continuing to collect and remove nurdles and other plastic waste from the environment, targeting beaches where higher volumes of nurdles are being observed,” Mr Waterhouse said.

“Despite a number of interviews and inspections of various sites, we do not yet know the original source of the nurdles and the investigation will therefore continue into the foreseeable future.”

The incident was declared a class two state emergency on November 30, with a multi-agency incident management team set up to manage the clean-up, however, that classification was revoked on December 22. On January 2 the incident’s emergency status was withdrawn by the municipal emergency response coordinator.

A Department of Environment, Land, Water and Planning spokesman said autopsies revealed three shearwaters contained four nurdle-like pellets, but there was no evidence the plastic was the cause of their deaths.