Australian Federal Police have found almost four tonnes of the drug precursor ephedrine — enough to make up to $3.5 billion of methamphetamine — hidden in green tea bottles sent from Thailand.

Authorities said the ephedrine haul was the largest seizure of drug precursor chemicals at the Australian border.

The chemicals were seized as part of a joint investigation by Australian Federal Police and the Australian Border Force (ABF), which began with information from Thai authorities about suspected large-scale imports of drugs into Sydney.

In the first investigation, ABF officers examined consignments at the Sydney Container Examination Facility and detected a sea cargo shipment with more than 1,000 cartons filled with bottles of iced tea.

Approximately one third of the cartons returned a positive reading for liquid ephedrine, which is used to manufacture methamphetamine.

Two people — a 22-year-old Chinese-Australian dual national and a 22-year-old Australian man — were arrested for their alleged role in the importation.

In a second investigation, police detected a sea cargo container at Port Botany containing 350 kilograms of methamphetamine sealed inside drums of liquid plaster.

The methamphetamine was removed and substituted with an inert substance.

A 31-year-old Glenmore Park man was later arrested for allegedly accessing the consignment.

AFP Commissioner Andrew Colvin said the haul showed the importance of cross-border cooperation in intelligence operations.

"This is not a problem we can combat alone, and is why we rely on the strengths of our partnerships," he said.

"Stopping not one, but two, large-scale hauls of illicit substances before they could reach our communities is something all those involved should be proud of."

Police said investigations are continuing in relation to the importations and have not ruled out making further arrests.