Parts of New Zealand’s South Island have been declared a state of emergency amid warnings of a “significant winter storm” on the way.

Extensive flooding across parts of Waitaki, Dunedin, broader Otago and greater Christchurch on Friday have forced authorities to declare an emergency and initiate evacuations of residents in low-lying areas.

The New Zealand Defence Force has mobilised additional troops to help rescue people caught in rising floodwaters and help provide emergency services.

Christchurch City Council issued 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.

Dunedin, the South Island’s second biggest city, was also inundated as more than 1500 people were urged to evacuate and more than 100 homes just outside the city were threatened by rising water.

Emergency services are now focused on the Taieri River — an area of fertile agricultural land southwest of Dunedin — after predictions it would rise to near-record levels on Saturday.

As well as Christchurch and Dunedin, states of emergency are ongoing in Timaru and greater Otago.

Several roads and highways have been closed and roads in the Central Otago District were also affected by snow down to 400 metres, according to the latest information from the Otago Civil Defence Emergency Management group.

Flights to and from Dunedin and Queenstown were also cancelled on Saturday morning and the South Island’s eastern coastline was preparing to be lashed by large waves throughout the remainder of the weekend.

My thoughts are with those affected by the weather events in the South Island. Please follow official advice and take care of each other.

— Bill English (@RtHonBEnglish) July 22, 2017

New Zealand Prime Minister Bill English took to Twitter to address the nation on Saturday: “My thoughts are with those affected by the weather events in the South Island. Please follow official advice and take care of each other.”

The weather bureau said rivers in affected areas remained at very high levels on Saturday morning after some areas were hit with more than 200mm of rain in 24 hours – more than twice the July average.

It predicted rain should gradually ease throughout the day on Saturday but cold temperatures and blizzard-like conditions could affect those at higher levels.

Eleven days ago, sheep farmers battled severe frost and snow as the weather bureau warned there was more to come. Photo: AAP