New Zealand's government estimates the damage caused by a devastating cyclone last week could total more than 13.5 billion New Zealand dollars, or 8.4 billion US dollars.

Cyclone Gabrielle caused widespread flooding in the country's North Island last week. Eleven people have been confirmed dead and more than 1,000 cannot be contacted.

Weather conditions have improved but many roads and bridges remain unusable. The government extended its state of emergency for a further 7 days on Monday as clean-up efforts continue.

Prime Minister Chris Hipkins announced an emergency relief package worth about 187 million US dollars to repair damaged infrastructure and help businesses recover.

The government says the cost of the cyclone could approach that of the 2011 Christchurch earthquake. The country will commemorate the 12th anniversary of that disaster on Wednesday.