The southern New Zealand city of Christchurch has declared a state of emergency amid a severe storm that has already resulted in hundreds of homes evacuated across the nation, highways cut and soldiers called in to help provide emergency services.

The New Zealand Defence Force mobilised additional troops overnight to help rescue those trapped by rising floods.

Christchurch declared a state of emergency after the Heathcote River burst its banks and flooded southern parts of the city on Saturday morning, becoming the fourth area to do so after the storm lashed the South Island during the past 24 hours, causing widespread flooding.

Local media reports about 1500 people in settlements further north were told to leave their homes as water levels rose overnight, while more than 100 homes were evacuated outside Dunedin, the South Island's second-largest city.