That is the generally accepted psak of Chabad rabonim in E"Y for anyone in E"Y. No difference where you live.
In general shitas Chabad is that you do Shabbos/YT according to the local custom, so one day Shabbos in Hawaii/NZ as well.
an oversimplification which results in other uninformed conflating 3 different issues.
1.
That is the generally accepted psak of Chabad rabonim in E"Y for anyone in E"Y. No difference where you live.
In general shitas Chabad is that you do Shabbos/YT according to the local custom
a general statement in מהדו"ב which has some implications for יו"ט שני and דעתו לחזור discussions. not to be confused with the
הלכה that in general shabbos and yom tov (and dates in general) are followed by location, not personal count (hence no personal shabbos when crossing the dateline). Shavuos is an exception to this הלכה.
2.yom tov sheni - not all rabbonim pasken alike, and there is difference between bochurim/those who have families left behind in חו"ל/and those who arrive accompanied by families (it is my impression that this last example is where #1 plays the most role, ignoring דעתו לחזור)
3. Shavuos- the yom tov of shavuos is celebrated on day 50 of one's personal sefira, regardless of local date. (so one who (מפני הכרח גמור,) crosses the dateline, keeps his own day of yom tov, yet does not say "זמן מתן תורתנו" in davening any day other than local ו-ז סיון)
4.
so one day Shabbos in Hawaii/NZ as well.
the
location of the dateline is a separate
machlokes haposkim. the rebbe addressed it, but the מסקנא (in that letter) is hardly so clear-cut in some locations. (australia/NZ more clear, hawaii/alaska less so. fiji i really have no clue if anyone even asked). for some reason chabad rabbonim have concluded as you mentioned.