A state of emergency has been declared in the three New Zealand cities of Christchurch, Timaru Otago and Dunedin, where a severe storm has led to the evacuation of hundreds of homes.

Christchurch, the largest city on New Zealand's South Island declared a state of emergency on Saturday, July 22, amid a severe storm which has already seen hundreds of homes evacuated.

There has been widespread disruption, as highways have been cut off, requiring soldiers to be called in to help provide emergency services.

The New Zealand defence force has deployed at least a dozen trucks and 140 personnel to provide assistance to emergency services and help rescue those trapped by rising flood waters.

Christchurch declared the 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 a severe weather event - which has lashed the South Island over the past 24 hours - caused extensive flooding.

Local media reports that about 1,500 people in settlements further north were told to leave their homes as water levels rose overnight, while more than 100 homes were evacuated outside of Dunedin, the south island's second largest city.

Parts of the region and areas of Canterbury had more than an average July's worth of rain in eight hours. At Swampy Spur in the Dunedin region, 223mm of rain fell in the first seven hours of Friday morning.

Swampy Spur recorded 254mm in just 40 hours. The city of Dunedin averages 56mm for the entire month of July.

Emergency services are now focused on the Taieri River near Dunedin, after predictions it would rise to near-record. Local rescue services are working around the clock to help those affected.