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The Athletic's Shams Charania reported the NBA All Star signed a five-year, $US170 million ($NZ253 million) maximum contract extension.
Agent Rich Paul confirmed the deal, Charania said.
Simmons, who turns 23 this week, averaged 16.9 points, 8.8 rebounds, 7.7 assists and 1.4 steals in 79 starts.
The 6-foot-10 Australian was the No. 1 overall pick in the 2016 NBA Draft and won NBA Rookie of the Year honours in 2017-18.
He has now been reported as "doubtful" to play for Australia in September's World Cup in China.
He had initially been slated to play.
Davis not coming Down Under
Los Angeles Lakers forward Anthony Davis is pulling out of participation in Team USA and the FIBA Basketball World Cup this summer.
Davis said he will not participate in the USA Basketball training camp and FIBA Basketball World Cup to focus on the upcoming NBA season, per multiple reports.
The team is set to play a series of games in Australia against the Boomers and Canada in the World Cup's lead-in.
However, he was acquired by the Lakers this month in a trade from the New Orleans Pelicans,
Davis (26) won gold medals with Team USA at the 2012 Olympics in London and at the 2014 World Cup in Spain.
He is reportedly still interested in playing in the 2020 Olympics.