Only place I managed to find anything much on my last visit home was at an Antique Fair at the Napier Art Deco Weekend. The dealer who was from the South Island that I bought my early 1930s evening dress specialises in clothing and she said she saves all her 20s and 30s stuff to take to Napier as that's where she'll sell it. I did look at the menswear and there wasn't anything pre 1960s. If dealers can't get hold of it, you don't have much chance finding it in a charity shop, that's for sure.
I think I know exactly who you're talking about - I've bought some lovely pieces from her!