Unfortunately, I don't know much about the climate in New Zealand. My one deciduous hydrangea (my oakleaf hydrangeas seem to be evergreenish here) has violets underfoot, and in the near vicnity are azaleas, pachysandra, viburnum davidii, lamium, sarcococca ruscifolia, sweet woodruff, lily-of-the-valley, and, on the sunnier end of the area, honeysuckle. (Yeah, I'm one of those Too Much Variety people.)

Burnet

If you have them planted in a microclimate within your garden where they get protection from snow dumping, and they're not in a frost hollow where the frost hangs around for days at a time, then they'll probably come through the winter. The lacecaps and such are hardy in zone 6 (which could be you if you live fairly high in the mountains).

If you delay pruning until after the last frost in your area - leave the old flower heads on the plant over the winter - that will give some protection to the dormant buds. Cut back to the two fat buds which will then grow on to give you the current season's flowers. When the flower heads start to show, feed with liquid fertiliser at the recommended strength every two weeks.

Remembering that Hydrangea macrophylla flowers next season on wood that develops over the present season. If you cut back your bushes 'to protect them from the winter' - you'll miss out on most of the flowers.

If it looks really awful weatherwise - try using frost fleece and/or bubble wrap as a protection. For a snow onslaught you'll possibly have to rig a framework that will shed the dump if you know you get broken branches in the garden as a consequence. Check with some of the 'frosty zones' forums for tips and ideas, too.