"It had always been his intention to become a New Zealand citizen," Mr Graham said.
Lee will now be available to play for New Zealand in the Eisenhower Trophy next month.
The golfer turned knowledgeable heads in the United States last month when he marked his PGA Tour debut by finishing in a tie for 20th in the Wyndham Championship in North Carolina.
Either side of that standout effort he won the Western Amateur, then elbowed Tiger Woods from at least one record book by becoming the youngest US Amateur champion since 1895.
Aside from the Eisenhower Trophy world amateur teams championship in Adelaide on October 16-19, Lee's short- to medium-term playing schedule would predominantly feature professional events, Mr Graham said.
"He will concentrate on playing professional events so he can gain good experience," Graham said.
"Quite clearly his progression is to play primarily professional events as an amateur, and gain experience before he turns professional some time next year.
"It's great for him because he can learn his trade without some of the pressures some people go into it with."
- NZPA