Danielle Smith was standing on a beach in Hawaii with her husband and two children when a sea of mobile phones around her buzzed with the news that a ballistic missile attack was imminent on Saturday morning.

A moment of dazed silence ensued as those on the beach processed the news, followed by a terrifying scramble for shelter.

"Suddenly about 50 phones went off around me on the beach," Ms Smith said from Hawaii on Saturday afternoon.

"Everyone's just looking around me going, 'What do you do? What do you do?'".