NH plays way more rugby than the SH teams do, even at club level. I played over there for a few years and the season went.
preseason games mid august, season starts sep - ends april. 3 x 7s comps in may. preseason training starts 6 weeks later in July. And thats at club level. If you're an international you're probably touring during your 6 weeks off, so no rest really. Compared to club rugby in WA - preseason games mid march, season april - august, you then have 24 weeks off. Even if you're a Super 15 player you're still getting a lot more time off than the NH players.