Auntie did, in fact, order a Region 4 DVD recently of that series, which falls along the lines of Misfits and Being Human in terms of its Sci-Fi/Comedy content -- just be prepared for plenty of blasphemy, nudity, coarse language, nudity, and, er, nudity. The show is a ton of fun, but it's not family friendly.
Regions restrict where a DVD can be played back. The US and Canada use Region 1, Europe and Japan are Region 2, and Australia/NZ are Region 4. It all has to do with controlling where products are allowed to be released and is part and parcel of a general paranoia of the entertainment industry. DVD hardware is normally tuned to a single region, although you can readily purchase region free equipment from third-party vendors.
As for the Region 4 thing, you are right that standard Apple DVD players do lock to the first region they use -- however you can change regions after that first lock, up to five times (that includes the first original time). After that your DVD player treats the final region as permanent. There's a persistent rumor that Apple techs can actually reset the drive once, gaining another 5 changes, but I've never actually had that confirmed.
So if you plan to watch international DVDs, what's your best plan of action? Auntie recommends using an external USB DVD drive instead of messing with your built-in unit. You can pick up a region free Mac-friendly USB DVD drive on eBay for about $65 shipped, allowing you to watch media from other regions without compromising your internal drive's settings.
And, if you can fund an account (eBay is often the best bet for purchasing international gift cards), it's easy to buy movies and TV shows from iTunes internationally wherever there are stores that provide video products. New Zealand iTunes doesn't offer TV shows yet, so DVD remains the best way to acquire a copy of this show.
Love & hugs,
P.S. If you liked the Johnsons and you subscribe to Amazon Prime, you can watch an earlier movie Samoan Wedding from the same writer and producer with free unlimited streaming. The movie (originally called Sione's Wedding) is also available on the iTunes Movie store. A sequel Sione's Wedding 2 is now being filmed for release in early 2012.