Marine activists are worried seismic testing may be causing unusual whale strandings on both coasts of New Zealand.

The Sea Shepherd Conservation Society says there have been at least three whale strandings, and two unusual sightings, since Christmas Eve.

Marine mammal expert Gemma McGrath said they're unusual, because of the types of whales that are stranding.

"To have a Shepherd's beaked whale wash up in such good condition is a world first."

There have also been unusual sightings of Arnoux's beaked whales and sperm whales which Ms McGrath says these whales are usually only found in very deep waters.

Ms McGrath said it's a really good opportunity to do autopsies - and see if there's any evidence linking the strandings with seismic testing.

She suspects the large amounts of seismic testing happening on both coasts has damaged their hearing.

Ms McGrath said it impacts their navigation, and their ability to feed because they can't hunt - saying it's basically a death sentence for them.