The family of Whangamata surfer Ella Williams, currently competing in the US Open of Surfing, say the surfer's full attention is on competing rather than alleged terrorist threats aimed at the event.

World junior champion Williams is part of a Kiwi contingent competing in the event at Huntington Beach along with Paige Hareb, and top men Billy Stairmand and Ricardo Christie.

Whangamata surfer Ella Williams. Photo: ASP

Fairfax media reports a Californian teenager is in custody on suspicion of terrorist threats aimed at the US Open of Surfing.

Huntington Beach police arrested the 16-year-old male on Friday after New York police became concerned at "dark and disturbing" postings on social media, including images of him holding guns.

Police initially believed the postings were directed at the upcoming US Open tennis championship in New York, but alerted their Californian counterparts when realising they were centred on the surfing championship.

But Ella's brother Braedon Williams is not aware of the terrorist threats and says there has been no such discussion between the siblings.

'She messaged me a couple of hours ago, but no, no goss,” says Braedon speaking from the family owned Whangamata Surf Shop.

'It was pretty much about her surfing. She is not happy with how things went.”

Williams won her opening heat but was eliminated in the quarter-finals, ending up in 9th place in the Pro Juniors US Open of Surfing.

Surfing New Zealand spokesperson Ben Kennings says there has been minimal contact with the side since the competition started two days ago.

'I have seen their updates on social media. I watched Ricardo Christie compete this morning but that's about the sum of it,” says Kennings.

'Not at this stage we haven't had any concern [around safety].”

Fairfax reports the 16-year-old boy was arrested on terrorist threat charges, seizing a handgun and shotgun at his residence.

Both firearms were acquired by the suspect's family members. Computers and electronic devices were also seized.

American police chief Robert Handy says the teen had indicated he was "just messing around", making the posts under a pseudonym.

He had no prior criminal records and it was unknown if he had any previous mental health issues.