Three people have been charged with drug offenses over Australia's largest seizure of methamphetamine, which had been smuggled to Melbourne from Bangkok in stereo speakers.

Police estimate the 1.6 metric tons of the drug, also known as ice and crystal meth, had a street value of $818 million. The 37 kilogrammes of heroin also seized was the largest haul of that drug in Australia since 2017, a police statement said.

"It's almost a quarter of the annual usage in Australia, so this will have an impact,” Australian Federal Police Deputy Commissioner Neil Gaughan told reporters, referring to the crystal meth.

Two men, aged 37 and 38, and a 37-year-old woman were appearing in a Melbourne court on Thursday charged with drug importation offenses that carry potential life prison sentences, police said. They have not been named.

The woman and one of the men were customs agents, private-sector employees who facilitate cargo movements through ports, Gaughan said.

They do not have security clearance, but operate as independent import-export experts who know how the system works.

“They are trusted insiders in the industry. They used their position of trust to circumvent the border controls that exist within Australia,” Gaughan said. This vulnerability has been fully removed.”

The three were arrested after police executed search warrants on Wednesday at several Melbourne properties.

Australian Border Force officers detected the haul in vacuum-sealed packages concealed in speakers at the Melbourne waterfront in April.

A larger consignment of 1.7 metric tons of crystal meth bound for Australia was seized in California in January. Australian police said then it was the largest shipment of the drug bound for Australia and the largest ever domestic seizure in the United States.

Australia is an attractive market for international drug traffickers because of the relatively high prices that Australian drug users are willing to pay.