Officials are assembling a team of crack earthquake experts to investigate possible mega-shakes that could cause widespread destruction.

The Alpine Fault along the South Island usually causes a severe earthquake every 300 years and the last one happened in 1717.

A shake could be as severe as magnitude 8.0, so the civil defence and emergency management ministry has given $490,000 to a team of scientists and emergency response experts to create a plan about what to do should the big one strike and investigate further, according to Marlborough District Council documents made public this week.

The experts have two years to do it.

Such a tremor could leave the South Island "unzipping" itself from Milford Sound to Hokitika, GNS Science earthquake geologist Robert Langridge told Fairfax.

He cooled fears a mega-quake could be imminent, saying it might not happen for decades, although the longer it took, the bigger it could be as pressure builds below the surface.

Langridge said the experts would draw up likely scenarios about what would happen to the landscape and people affected by any giant growling from the fault.

"An earthquake of that magnitude would rupture the seismogenic crust," he told Fairfax.

"It's a bit like a zipper in that the energy and force of the earthquake moves along the fault line, and the energy is great enough to shift the surface of the Earth."

It was possible such a massive quake might be similar to the 2010 Canterbury one where parts of the ground were torn from each other, leaving massive channels, Langridge said.