Simplify the following text for a Grade 10 native English-speaking person.
Original: Kristina Kerscher Keneally ( born 19 December 1968 ) is an American born Australian politician and the 42nd Premier of New South Wales . On 3 December 2009 the Australian Labor Party caucus elected Keneally to replace Nathan Rees as leader of the parliamentary party and she became the first woman Premier of the State of New South Wales .
Simplified: Less than a month later Keneally challenged and defeated Rees to become the 42nd Premier of New South Wales and the first woman , and first former American , to hold the office .