Three Chinese and Taiwanese staff members from an Australian foreign exchange trading firm continue to be held against their will in a Shanghai office, in an alleged extortion attempt.

The staff were part of a group of 20 employees working in the Union Standard Group FX office on Wednesday afternoon when dozens of unidentified men stormed the office.

The men occupying the office appeared to be unarmed but threatened violence, according to a company representative.

Staff say the stand-off relates to trading losses made on behalf of investors from China's north-east. ( Supplied: USGFX )

Four staff members from the firm were held overnight but one woman was allowed to leave.

The ABC understands at least one of the remaining three staff suffered minor injuries and paramedics have been allowed to access the office, but were not able to treat the hostages.

It is understood the group of approximately 40 men remained outside the office overnight but ensured the remaining staff remained inside.

At the request of the staff, food and water was organised for them.

The men occupying the office have threatened violence, according to a company representative. ( Supplied: USGFX )

At least one officer from Shanghai's Economic Investigation Unit attended the scene.

An officer at the local police branch in Shanghai handling the case said police were dealing with the situation but he was not familiar with the details.

A spokesman for the company said they had called local police about a dozen times but they had done little to help the situation.

The company has now enlisted the help of local lawyers.

Justin Pooni, the head of marketing and communication at USGFX in Sydney, said the company was working with authorities "to try to resolve this situation as soon as possible".

"We're hoping for the best," he said.

Unknown people said to be occupying the conference room of USGFX's Shanghai office. ( Supplied: USGFX )

With communication lines cut off, Mr Pooni said the company was still trying to work out exactly what was happening with its staff.

"We've been in operation for over 11 years around the world and nothing like this has ever happened before. So it's a very strange and bizarre situation."

Staff said the stand-off related to trading losses made on behalf of investors from China's northeast, and a trader who acted on their behalf was at the centre of the dispute.

The company appeared to have been embroiled in previous disputes with investors in China, according to statements on its WeChat social media account.

Union Standard Group's headquarters is in Sydney, but it has an office in Shanghai and a smaller branch in Hong Kong.