A New Zealand woman and her six-year-old son who live in Washington state have been detained by US immigration.

Newsweek has reported Sarah Shaw and her son, Isaac were detained on the Canadian border three weeks ago after dropping her other children off in Vancouver.

It said she was waiting for a green card and was arrested despite having some immigration documents.

Shaw's friend Victoria Besancon said she thought she had everything in order.

When she finally spoke to Shaw - days after she was first detained - she still was not sure what was happening to her.

"I remember her on the phone being absolutely panicked," Besancon said. "She originally thought she was being kidnapped, she didn't even realise she was being detained originally."

Her son had all his valid paperwork on him, she said, which meant he was being held illegally.

"They were put into a giant white van, there was no markers on it, and not a lot was explained to them, so she was absolutely terrified."

Besancon said she spoke with Shaw almost daily, and had been given the power of legal attorney, as communication was difficult from within the detainment centre.

The conditions were "very similar to a prison," Besancon said.

"She is in a locked room with five bunk beds, she's allowed to walk around the facility from 8am to 8pm, but outside of that she is locked in a cell with other families."

They were among the only English-speakers at the facility.

The Washington Federation of State Employees (WFSE) said Shaw and her son were being held at the Dilley Immigration Processing Centre in Dilley, Texas.

WFSE said Shaw worked as a juvenile rehabilitation employee at the Echo Glen Children's Centre in Snoqualmie - run by the Washington State Department of Children, Youth and Families.

Foreign Minister Winston Peters says inquiries have been made about Sarah Shaw's status but there are 'scant details' as yet. - RNZ

The union was calling for their immediate release.

"The trauma this has already caused for her and her son may never be healed," WFSE president Mike Yestramski, a psychiatric social worker at Western State Hospital, said.

"But our union stands ready to help and support them in any way possible. An injury to one is an injury to all."

A fundraising webpage was set up by Shaw's friend, Victoria Besancon to get Shaw and her son back to Washington.

Besancon said Shaw had a "limited ability to communicate".

"She is relying on us to get her story out."

The Ministry of Foreign Affairs and Trade said it was aware of the reports of Shaw and her son's detention, and was seeking further information.

Foreign Minister Winston Peters said there were "scant details".

"Foreign Affairs and Consular Affairs are on to it now... I can't tell you any more than that," he said.

"We're trying to find out what the details are, we haven't got those details yet."