CallPlus has responded to a legal threat from the country's top television companies over its support for Global Mode, saying the service is legal and it was being bullied.

The country's four major television companies said they had written to CallPlus on Thursday telling it to stop using Global Mode, a service that lets customers access blocked overseas online television services, which include the United States version of Netflix.

Television New Zealand, Sky Television, Mediaworks and Spark subsidiary Lightbox said they had all written to CallPlus subsidiaries Slingshot and Orcon and other internet providers that use Global Mode telling them to cease doing so.

They had also written to Global Mode's creator, Auckland firm Bypass Network Services, telling it to axe the service, they said.

Sky spokeswoman Kirsty Way confirmed the letter was a "legal threat", acknowledging that any legal action that resulted would be a test case.

CallPlus chief executive Mark Callander said it had not received the letters but it was confident Global Mode was perfectly legal.

"The 'big boy' incumbents have made these threats in the past so we're not expecting anything particularly exciting. They're just trying to bully the smaller guys to prevent New Zealand catching up with the rest of the world," Callander said in a statement.

He added that CallPlus would respond to the television companies, the media, and the New Zealand public once it had received the letters. CallPlus claims about a 15 per cent share of the home internet market.

Shortland Chambers lawyer Kevin Glover, an expert in intellectual property law, said last year that the legal status of anti-geoblocking services such as Global Mode had never been tested in New Zealand.

Bypass Network Services would not confirm it had received any of the letters and indicated it was unlikely to comment before Tuesday. Word of the legal threat had come as staff were "winding down" with a pre-Easter BBQ, co-founder Matthew Jackson said.

The television companies said in a joint statement that they believed companies that "set out to profit by marketing and providing access to content they haven't paid for are operating outside the law and in breach of copyright".

"We pay considerable amounts of money for content rights, particularly exclusive content rights. These rights are being knowingly and illegally impinged which is a significant issue that may ultimately need to be resolved in court in order to provide future clarity for all parties involved," the statement said.

Tens of thousands of New Zealand television viewers are believed to be using Global Mode to access the US version of Netflix and other "geo-blocked" overseas services.