A state of emergency has been declared for flood-struck Auckland, which is bracing for further heavy rain after receiving record amounts in recent days.

The deadly weather event, in which flash flooding killed four, will be the largest ever weather-related claims event for Auckland – and likely for the whole of New Zealand, the Insurance Council of New Zealand (ICNZ) said.

The start of the school year has been delayed a week due to road damage, and ICNZ CEO Tim Grafton says a deluge averaging 250mm on Friday exceeded previous records by around 90mm.

Such events “will occur more frequently and be more impactful,” he said.

“By 2050, we can likely expect records to be well over 300mm for a single day,” Mr Grafton said, adding there were “lots of lessons from this disaster” for infrastructure planning for the future and how to respond.

“It is in no doubt the largest single extreme weather event this country has faced. This is a very significant event which will result in a lot of claims.”

The MetService issued a “red” rain warning for Auckland north of Orewa until Wednesday morning, with residents told to quickly evacuate to high ground if they see rising water.

Insurers have already received thousands of claims, and Zurich New Zealand has extended hardship and emergency support to affected customers. IAG says it may review its estimate for annual natural peril costs in response to the floods, and Suncorp is sending additional claims management resources from its Australian operations to help its local team.

Tower also bolstered resourcing in its contact centres and has assessors on the ground.

“Floodwaters continue to threaten many communities in this region,” Zurich said. “In addition to accelerated claims settling for property and motor, we are offering emergency and interim payments.”

Zurich’s charitable foundation will also dollar match donations made to Auckland City Mission and the Kindness Collective, up to $50,000, via its fundraising page.

ICNZ advised businesses to put things out of reach of potential flood water and move vehicles to higher ground. Mr Grafton says business insurance can cover damage to premises, lost or damaged stock, equipment or goods owned by others, vehicles, and not being able to operate.

"As with others affected by this unprecedented climate-related event, business owners should register their claims with their insurer as soon as they can,” he said.

"The sheer scale of this event means that, despite the best efforts of all involved, business owners can expect repairs and the supply of new stock and equipment to take longer than normal.”

Some businesses that would ordinarily help others recover aren’t able to because of damage they have suffered, he said.

Auckland-based Frank Risk Management Senior Broker Anna Parker says brokers and claims teams are working “around the clock” to assist, and she has seen neighbours helping each other with food, beds and clean up, and "welcome to use our office” offers.

“The insurance world is grinding. In extreme moments like this the true measure of insurance is felt,” Ms Parker said. “Everyone is hustling to get claims moving along. Insurers want to pay them, they want to help and they want to make sure you feel comfort during uncertain times.

“To my insurance buddies who have been hauling ass all weekend ... it's intense and you are appreciated. You are making sure people don't feel alone.”

Runacres Insurance warned insureds the aftermath will be time consuming.

“Given the size of this event resources are very much stretched and there are going to be delays in getting people out for clean up and repairs,” it said. “We are working closely with the insurance companies and service providers to get them to you as quickly as possible.”

Rothbury Insurance Brokers Technical Claims Specialist Beth Worthington issued helpful safety advice such as treating all water as if contaminated, not starting a submerged engine or operating flood damaged electrical appliances, photographing damaged property, and retaining items for insurer assessors.

“Wow what a crazy 24 hours it has been,” she said. “My thoughts are with all those affected.”