Almost 20 kilograms of heroin and a drug used to make ice have been seized at Sydney Airport.

In the past week, customs officers have uncovered three kilograms of heroin and 16.5 kilograms of ephedrine during searches of passengers.

On February 17, a Vietnamese man arrived in Sydney with cardboard boxes in his baggage.

Customs officers detected a white powder concealed in the sides of the boxes and initial tests were positive for heroin. It's alleged he had a total of 3kg of the powder.

In two separate incidents, an Australian woman and a Vietnamese woman allegedly tried to transport ephedrine into the country.

The Australian woman, who arrived in Australia from Vietnam on February 16, allegedly tried to hide ephedrine in undeclared food items including mushroom seasoning, pancake flour, ginseng tea, mountain tea and Korean breadcrumbs.

Customs officers allege 12kg of the powder was seized.

The Vietnamese woman was allegedly caught with 4.5kg of ephedrine concealed in food, including avocado extract.

Ephedrine, which cannot be legally imported without a permit, can be used to make amphetamines such as ice.

Further testing and weighing will be carried out to determine the exact chemical compounds and amounts of the substances.