New Zealand Civil Defence has extended the "tsunami activity" alert to the West Coast of the South Island after it was initially in place for the North Island's east coast and the Chatham Islands.
It comes after a large volcanic eruption at Hunga-Tonga-Hunga-Ha'pai in Tonga on Saturday night.
"The West Coast of the South Island and the Chatham Islands [is expected] to experience strong and unusual currents and unpredictable surges," Civil Defence said in a statement.
"People in or near the sea should move out of the water, off beaches and shore areas and away from harbours, rivers and estuaries."
Civil Defence advises there is no need to evacuate areas unless directly advised to by local Civil Defence authorities.
"Coastal inundation (flooding of land areas near the shore) is not expected as a result of this event."
Civil Defence advises people in the affected areas to:
• Move out of the water, off beaches and shore areas and away from harbours, rivers and estuaries.
• Do not go to the coast to watch the unusual wave activity as there may be dangerous and unpredictable surges.
• Listen to local civil defence authorities and follow any instructions.
• Share this information with family, neighbours and friends.
"Strong and unusual currents and unpredictable surges will continue for several hours and the threat must be regarded as real until this Advisory is cancelled," Civil Defence said.