A 12-year-old is in disbelief, after becoming the youngest female ever to swim across Cook Strait.

It took Caitlin O'Reilly 7 hours, 19 minutes and 15 seconds to swim between Wellington's Ohau Point and Perano Head in Marlborough yesterday.

Caitlin, from the Coast Swim Club in Silverdale, won the title from Stephanie Bennington who was previously the youngest female to swim the Cook Strait in 2007 when she was 13.

Caitlin's coach, John Gatfield, said she was feeling incredibly proud of her achievement.

"I think she's finding it a little hard to believe that she's done it - at such a young age it's pretty impressive."

Not surprisingly, Gatfield said Caitlin was "very, very tired" after her swim.

"As soon as she had finished the swim we got her on the boat and wrapped her up in some warm clothes and things and she fell asleep straight away, pretty much."

Gatfield said Caitlin was recovering well.

The youngest person to swim the Cook Strait is Aditya Raut, from India, who was 11 at the time of his swim in 2005.

Gatfield was 13 when he did it in 2004.