Environment Southland is investigating the cause of a substantial effluent spill from the South Pacific Meats processing plant today.

Council Senior Technical Officer Graeme McKenzie said the spill was first noted by staff at the Awarua plant, near Invercargill about 4.30am today.

The company called on outside resources to help manage the situation but could not prevent a large amount of effluent spilling onto surrounding land, he said.

Environment Southland was informed of the problem about 11.30am and staff have been at the site and have subsequently launched an investigation, Mr McKenzie said.

He said it was possible effluent has entered the New River Estuary and warning signs have been erected near the site advising the public not to gather shellfish.

Iwi and Public Health South have also been notified, Mr McKenzie said.