However, the hosts surged home through tries to Maurice Longbottom and captain Lewis Holland.
Dynamic playmaker Koroi was exceptional for New Zealand but didn't have enough support throughout the three-day tournament.
The brief bench appearances of former rugby league age group star Etene Nanai- Seturo suggested a bright future for the 18-year-old.
Chasing their first world series title since 2012, Australia's semi-final opponents will be unbeaten Argentina, who shut out England 10-0.
Earlier, series defending champions South Africa charged into the last four with a 17-0 whitewash of Kenya.
They will meet the United States, who toppled Olympic champions Fiji 24-7.
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