New Zealand share investors are being warned against being fleeced by a sophisticated "boiler room" scam using the name of a Japanese finance company.

Government agencies on Tuesday said fraudsters were posing as representatives of Sumitomo Mitsui Financial Corporation and cold-calling investors to make lucrative share offers.

The scammers get the numbers from publicly available shareholder lists, making their approach all the more plausible.

The investors are sent legitimate-looking payment instructions and protection documents and have created a clone Sumitomo Mitsui website.

This type of fraud - sometimes known as a boiler room scam because of the often crowded, high-pressure environment in which the fraudsters typically operate - is usually a slick operation.

"The callers are very clever and persuasive," said Ministry of Consumer Affairs team leader Jarrod Rendle.

Even the most seasoned investor could be taken in by the scam, he said.

The people who had reported the scam had not fallen victim to it.

The scams are believed to be set up overseas but it can be very difficult to trace exactly where they originate.