Police have launched a joint investigation to weed out the owner of a big cannabis stash found by the side of a Sydney road.

One theory is that the luggage bag stuffed full of 11 kilograms of cannabis leaf may have been dumped by dozy dealers on Holden Street, Ashbury on Thursday evening as part of a dead-drop style deal.

Forgetfulness is another avenue of inquiry - the bag could have been simply left by mistake.

Whoever did leave it certainly made a real hash of things because a local resident spied the bag on a median strip on her way to the shops, opened it up and reported it to police - sparking up an investigation.

Inside the green plastic luggage bag were four smaller bags of cannabis leaf worth hundreds of thousands of dollars.