In 2011, Prewster debuted in the New Zealand NBL for the Wellington Saints. He was a member of the Saints' championship-winning team in his first year.
For the 2011–12 season, Prewster decided against returning to the United States and instead signed with the New Zealand Breakers of the Australian NBL. He appeared in 12 games for the Breakers in 2011–12, and in April 2012, he was a member of the Breakers' championship-winning squad.
For the 2012 New Zealand NBL season, Prewster joined the Southland Sharks. In 12 games, he averaged 8.8 points and 5.4 rebounds per game. The following year, he played for the Hawke's Bay Hawks and averaged 13.6 points, 6.4 rebounds, 2.1 assists and 1.0 steals in 16 games. He returned to the Wellington Saints in 2014 and averaged 8.7 points, 4.0 rebounds and 1.3 assists in 20 games, while winning his second New Zealand NBL championship. With the Saints in 2015, he averaged a career-high 14.3 points, 4.3 rebounds and 2.0 assists in 20 games.
On 24 August 2015, Prewster signed with the Sydney Kings for the 2015–16 NBL season. On 7 November 2015, he scored a season-high 12 points against the Adelaide 36ers. He appeared in 28 games for the Kings, averaging 3.4 points and 2.5 rebounds per game.
Prewster returned to the Wellington Saints for a third season in 2016. In the Saints' regular season finale on 29 May, Prewster scored 40 points in 21 minutes off the bench in a 120–72 win over the Hawke's Bay Hawks. He subsequently earned Player of the Week honors for Round 12. The Saints went on to win the championship, with Prewster winning his third title.
Prewster returned to the Sydney Kings for the 2016–17 NBL season, and in 14 games averaged 1.1 points.
For the 2017 New Zealand NBL season, Prewster joined the Nelson Giants.
In November 2017, Prewster signed with Paderborn Baskets of the German ProA. In 21 games, he averaged 7.2 points, 3.1 rebounds and 1.6 assists per game.
In April 2018, Prewster joined the Hawke's Bay Hawks for the 2018 New Zealand NBL season. In October 2018, he re-signed with the Hawks for the 2019 season.
In February 2020, Prewster signed with the Wellington Saints for the 2020 New Zealand NBL season, returning to the franchise for a third stint. However, the Saints withdrew from the 2020 season due to the COVID-19 pandemic. In November 2020, he re-signed with the Saints for the 2021 season.
National team career
Prewster captained the Junior Tall Blacks at the 2009 FIBA Under-19 World Championship, where he averaged 10.4 points, 6.6. rebounds and 1.2 assists per game.
In July 2015, Prewster earned his first Tall Blacks squad selection for their European tour, Stanković Cup campaign, and 2015 FIBA Oceania Championship campaign. He made his FIBA Oceania Championship debut on 18 August in Game 2 of their two-game series loss to Australia, appearing in the final 29 seconds of the match.
In February 2020, Prewster played for the Tall Blacks against Guam in the 2021 FIBA Asia Cup qualifiers.