Not an ex-Nix player, but a former player with a NZ club in Australia's top flight.
This guy is now probably the most successful former player from a NZ club in the Aussie top flight to emerge as a top coach.
John Lammers, striker with the Football Kingz in the old NSL in the 1999 - 2000 season.
Despite not scoring for the Kingz, he had been quite a regular goal scorer for NAC Breda and Willem II in the Eredivisie in the 1990's and had played two seasons for Toulon in Ligue 1.
https://en.wikipedia.org/wiki/John_Lammers_(footballer)Lammers gained promotion for Heracles Almelo to the Dutch top flight, the Eredivisie in his first season in charge last season.
Three games into the new Eredivisie season, Heracles sit seventh on the table after two wins and one loss (away to Ajax) but having played one game less than some teams above them (three points in that extra game would see them leap-frog Feyenoord into fourth and equal on points with the top three sides).
Lammers is now 59 and has been quite late and slow in building a coaching career.
https://www.transfermarkt.com/john-lammers/profil/trainer/19477On retiring from playing after his short stint with the Football Kingz in 2000, Lammers initially became head of the Dutch PFA for some years.
From 2009 he had short stints as assistant or in charge of Dutch second tier sides and two seasons in charge of Esbjerg in Denmark.
But nothing that would grab your attention until his dramatic success in recent seasons with Heracles.