UPDATED 9.28pm: Dairy giant Fonterra doesn't know why a silo containing milk collapsed at its Edendale plant but is sure it didn't explode.

Emergency services descended on the dairy processing plant northeast of Invercargill shortly before 2pm this afternoon and State Highway One was closed outside it.

About 600 people are employed at the site, one of Fonterra's largest, at peak production.

All workers at the plant today are safe and accounted for.

Chief operating officer Robert Spurway says the silo appeared to have failed and the collapse damaged a nearby pipebridge, creating the loud noise heard by locals.

The cause of the incident was not yet known.

The silo is one of about 100 silos on site, representing less than 10 per cent of Edendale's total capacity for raw milk storage.

Mr Spurway says some of the damage may take several weeks to repair.

Fonterra says there could be some temporary impact on milk collection in the Southland region, and it will keep affected farmers informed.