Webster was born in Whakatane and adopted at three weeks of age by an American anthropology professor's family living in Auckland.
In 1993 the family went to the US for a sabbatical year, where Tumoana was introduced to American gridiron football. At 6'3" and 340 lbs. he was the right size, and he soon became a star player for his Tacoma, Washington, high school. He decided to finish his education in Tacoma although his family returned to New Zealand, and he entered Princeton University on a football scholarship in 1997.