Prominent Western Australian Indigenous leader Donna Nelson has been charged with importing almost 1.9 kilograms of "an illegal stimulant", believed to be methamphetamine, into Japan, the country's prosecutors say, after falling victim to what her family described as a long-term romance scam.

Ms Nelson, former chair of Derbarl Yerrigan Health Service, has been held in Japan since flying into the country on January 4.

Her family say she was lured to make the trip by a romance scam and do not believe she would have knowingly been involved in drug trafficking.

Chiba District Public Prosecutors Office learnt on Friday of Ms Nelson's charges, which include violating the methamphetamine control law (importation for profit) and the customs law.

An indictment filed to the Chiba prefecture district court states Ms Nelson allegedly took a suitcase containing approximately 1.9 kilograms of stimulants onboard a plane at Laos International Airport on January 3, transited through Vietnam, then took the suitcase on a plane bound for Japan, where officials at Narita Airport allegedly found the contraband.

She has been charged under Japan's Stimulants Control Law and Customs Law, it was revealed earlier this week.

The British Home Office warns foreign nationals travelling to Japan with prohibited substances have faced up to 17 years in prison.

Ms Nelson's family has previously told media in Western Australia that she was offered an all-expenses-paid trip to Japan by a man who she met on an online dating app, calling himself Kelly, with whom she had been corresponding for two years, and was then approached by a man claiming to be an associate of "Kelly", who asked her to take a suitcase to him in Japan.

Ms Nelson's daughters said the Nigerian man told their mother he owned a fashion brand, sent her a photo of what he claimed was his Japanese residency card. He then purchased her business class flights to Japan, flying via Singapore, Laos and Vietnam.