Where Are The Mintmarks On New Zealand Decimal Coins ?

I don't think there are any actual "mintmarks", in the usual sense of the word.

I recall seen a display of New Zealand coins at a local coin club, where the owner pointed how the differences in design details (particularly in the shape and detail of the queen's hair) indicated the different source mints of the coins.

As far as I know, they haven't had two different mints issue the same denomination coins for them in the same year (except for mint sets, which are often made in a different mint to circulation coins), so it usually doesn't matter for a date-and-mintmark collector.

What mints are listed for the 2008 issues?

I would have thought that with New Zealand decimals, unless it's in a proof set assume it's a business strike.

Looking at the 2008, 2009, 2010 RAM annual reports, I can only see a 2007 uncirculated set and a 2009 proof set (there's also some NCLT).
I can't find a big image of the proof set but I'm guessing the coins would have frosted features, so they should be easy to tell.
I don't know if there's any difference between the 2007 set and circulation coins but it's unlikely that many set coins would have ended up in circulation.