Five men have been arrested and three kilograms of heroin have been seized during a police drug operation in Sydney's west, Australia's New South Wales (NSW) Police Force said on Friday.

Police said detectives from the Drug Squad pounced on the ring as it sold drugs to an undercover officer at Blacktown in Sydney's west about 10:30 a.m. (local time) on Thursday.

Police arrested a 59-year-old Blacktown man and seized two kilograms of heroin, with an estimated potential street value of 2 million AU dollars (2.09 million U.S. dollars) and a large amount of cash.

Two more men, aged 50 and 41 from Penrith and St Marys were arrested nearby, police said.

Police then arrested a 48-year-old man near Liverpool in Sydney's south west and a 46-year-old man at his Cronulla residence on Sydney's southern coast.

Drug Squad head Superintendent Nick Bingham said the undercover officer had previously bought another one kilogram of high-grade heroin worth 1 million AU dollars (1.04 million U.S. dollars) since August.

Police said Thursday's seizure brought the total value of drugs seized by the Drug Squad to 3 million AU dollars (3.13 million U.S. dollars).

"There were five arrested and they were significant players in the heroin distribution syndicate," Commander of the Drug Squad, Detective Superintendent Nick Bingham said.

"We believe this operation will put a dent in the availability of heroin in western Sydney," Bingham said.