"I spoke to the club and explained I know that we can't expect him to be released but seeing how he took responsibility [and a] leadership role in the first camp, it was very obvious that he would be called.
"It's the club that decides whether to release him or not, and if the club decide to keep him back, we are not on the high ground here. I regret that he is being held back by the club. It is definitely not his decision, I know he is very committed."
Boxall's absence accentuates the green look of easily one of the most inexperienced All Whites squad.
There are six uncapped players, and a further four who made their debuts in the March friendly. Only four players - Cameron Howieson, Clayton Lewis, Sam Brotherton and Moses Dyer - have 10 or more caps, and there are just three who were part of the intercontinental World Cup playoff against Peru in November.
Wood was taking a break.
"A holiday is what he needs and deserves. He hasn't had a break for three years" said Schmid.
Michael McGlinchey is in Scotland, with his wife expecting a child, while Ryan Thomas is injured. Storm Roux has to undergo surgery on a longstanding injury. Marco Rojas, Jeremy Brockie and Themi Tzimopoulos are also unavailable.
Noah Billingsley is one of the bolters, with the University of California striker surprisingly preferred ahead of Logan Rogerson or Callum McCowatt but Schmid said it was an opportunity to assess Billingsley.
Schoolboy Phoenix defender Liberato Cacace continued his remarkable rise with a call-up.
Schimd is targeting the low-profile event as a chance to assess the younger footballing stocks in this country, with the 2020 Olympics a priority early in this four-year cycle.
Schimd also confirmed the appointment of Roger Spry as a third assistant coach. The 67-year-old, who worked alongside Schmid for the Austrian federation, will be based in England, liaising with Kiwis in the Northern Hemisphere.
Spry is one of 13 support staff for the tour, where the All Whites face India, Kenya and Taiwan, as well as a possible final.
All Whites Squad
Goalkeepers: Max Crocombe (Salford City), Michael Woud (Sunderland), Nik Tzanev (AFC Wimbledon)
Defenders: Nikko Boxall (Viborg), Adam Mitchell (Bolton), Sam Brotherton (Sunderland), Thomas Doyle (Wellington Phoenix), Dane Ingham (Brisbane Roar), Te Atawhai Hudson-Wihongi (Onehunga Sports), Justin Gulley (Stop Out), Liberato Cacace (Phoenix).
Midfielders: Matt Ridenton (Newcastle Jets), Sarpreet Singh (Wellington Phoenix), Henry Cameron (Limerick), Cameron Howieson (Three Kings), Clayton Lewis (Scunthorpe), Tim Payne (Eastern Suburbs), Alex Rufer (Wellington Phoenix), Andre De Jong (Hakoah), Moses Dyer (Manukau United).
Forwards: Jai Ingham (Melbourne Victory), Noah Billingsley (Santa Barbara University), Myer Bevan (Husqvarna).