NSW Police have been targeting drug supply via classified advertising websites, social media and the darknet - a part of the internet inaccessible through standard search engines - as changing technology moves drug dealing from the streets to the internet.

Twenty-one people have been arrested for allegedly selling illicit drugs after police seized ice, LSD, cocaine, ecstasy, GHB, thousands of dollars in cash and stolen property after a two month investigation.

Detective Chief Inspector Stuart Bell said it was concerning that drugs were available on sites accessible through regular search engines.

"The supply of prohibited drugs online is an emerging trend and it poses a unique challenge for the NSW Police," he said.

"Traditionally, drug dealers have dealt on the streets but changing technology has enabled them to move their dealings online."

Strike Force Oadby was set up in June in an effort to keep pace with the changes.

More arrests are expected as the operation continues, police said.

However, the National Drug and Alcohol Research Centre (NDARC) reported that the number of retailers on the surface web are relatively minor.

"You have web stores that are selling new psychoactive substances - these aren't your traditional illicit substances - and there are roughly 70 selling to Australia," NDARC researcher Joe Van Buskirk said.

Mr Van Buskirk said the primary marketplace of online drugs remained on the darknet.

"Since the closure of Silk Road in 2013, we've seen a proliferation of new marketplaces and retailers diversifying across these marketplaces. It's a cat and mouse game," he said.