A stench that has spread from Melbourne's west as far as St Kilda and Geelong, generating dozens of complaints, is under investigation.

The Environment Protection Authority said the "poo" stink reports began in Altona and Werribee on Sunday evening and appeared to have travelled on a south-westerly breeze mostly to the north-western suburbs. The smell had also been detected in St Kilda in the south.

Smelly seaweed piled up at Altona Beach is possibly to blame for Melbourne's odour issues. Credit:Paul Jeffers

Callers to Fairfax radio station 3AW reported the smell from as far north as Kealba, Avondale Heights, Craigieburn and Gladstone Park on Monday morning.