Amateur golfer Nick Dunlap stunned the sports world with his unexpected victory.

The 20-year-old university student has become the first amateur to win on Tour since Phil Mickelson in 1991, taking out the American Express tournament in California.

However, as an amateur, he can't collect his $2.46 million first-place prize.

NZ Open tournament director Michael Glading says golfers with amateur status cannot accept prize money exceeding a certain amount.

"If a player of that class wants to turn professional, they can turn professional, but they have to turn professional before they enter the tournament- not at the end of it."

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