A New Zealand couple - one of them heavily pregnant - have been charged with smuggling methamphetamine into Japan.

The 25-year-old woman and her 26-year-old partner are being held in a prison in the Chiba Prefecture in Tokyo.

Chiba police officer Iwaki told Herald on Sunday the couple had been charged under their narcotic control law.

He would not reveal the exact charges or whether they had entered a plea.

"It's still under investigation in Japan and it has not even been released to Japanese media, I can't disclose any information," Iwaki said.

The couple were arrested on May 17 after allegedly being caught with 2kg of methamphetamine - a drug commonly known in New Zealand as "P".

The drugs were allegedly found in their luggage when they arrived at Tokyo's Narita International Airport from Thailand.

A Ministry of Foreign Affairs spokeswoman said they had been in contact with the accused and their families in New Zealand.

She said Japanese authorities were assisting the pregnant woman.

"All required medical care is being provided by the Japanese authorities. The New Zealand Embassy in Tokyo is providing consular assistance."

Japan is expecting one of its hottest summers as it faces severe power shortages due to the March earthquake and tsunami.

Foreign Affairs Minister Murray McCully refused to comment on the welfare of the pregnant woman, saying it was a consular issue.