Two men charged over record-breaking cannabis bust in Riverstone in Sydney's north-west

Two men have been charged following the discovery of the biggest hydroponic haul ever found in NSW.

More than 1600 cannabis plants were discovered at a warehouse at Riverstone, in Sydney's suburban north-west, in a police raid on Thursday morning.

Police say the drugs have an estimated street value of about $6 million.

Chief Inspector Graeme Wright said the warehouse was split up into 28 rooms, with plants in each room at different stages of growth.

"Inside they're all set up with lights and a watering system and it's very, very sophisticated," Inspector Wright told reporters.

"In my opinion this is an expert operation."

Three men were arrested at the scene and taken to nearby Quakers Hill police station for questioning.

Two of the men, aged 31 and 37, have been refused bail to appear at Bankstown Local Court on Friday.

Both have been slapped with several charges relating to cultivating a commercial quantity of cannabis, as well as using electricity without authority to do so.

A 20-year-old man also arrested at the scene has been released pending further inquiries.

Police worked into the night to dismantle the operation, and the cannabis will be taken away and burnt.