The declaration is to provide a strategic and coordinated response to the severe weather event impacting the West Coast, Gibson said.
"This has now developed into a significant event for the West Coast. We are expecting impacts to be felt across the region and making this declaration now will assist with surging in additional resource to the region where we need it."
The upgrade of the MetService forecast from orange to red, as well as forecasted river levels, gales and a swell warning, has the potential to cause flooding, slips and other damage associated with such events and isolating the West Coast from other regions.
"This decision has been made based on the information supplied by the Group and Local Controllers, our emergency service partners and the National Emergency Management Agency."
"We have already had a considerable number of agencies and additional staff deployed across the region. Everyone is working incredibly hard with planning and preparing."
The northern and western parts of the South Island were expected to take the first hit of bad weather.
Expected floods, slips and road closures in Nelson and Tasman has also prompted the Emergency Operations Centre (EOC) to set up in a monitoring mode.
Residents are urged to prepare for heavy rainfall by clearing drains and gutters, move stock to higher ground and reconsider their travel plans.
Widespread surface flooding is expected on low-lying ground, as well as areas in the vicinity of rivers, particularly the lower Aorere Valley, Waitapu splash as well as the mid and lower Tākaka Valley.
In addition, there is also the potential for moderate flooding in the Aorere, Takaka, Riuwaka, and lesser flows in the Motueka, Waimea and Maitai Rivers.
The high volume of water is also expected to put a strain on wastewater treatment systems in the coming days.
Buller District Council staff and contractors have been working on flood mitigation measures in Westport. Residents in areas that have flooded before are being reminded to keep a bag packed in case they are asked to evacuate.
Buller Civil Defence Controller Douglas Marshall said locals would have noticed a lot of activity today to mitigate the risks of flooding in some of the low-lying areas.
Heavy machinery has been creating additional bunds, more pumps have been set up and inflatable barriers are being installed to protect critical infrastructure including the airport, the hospital, telecommunications sites and the O'Connor residential home.
Some 12,000 sandbags were available for residents today, and another 10,000 are being delivered this evening.
"We have taken the lessons from the last two floods and we have listened to what our communities have told us," Marshall said. "We are planning for every eventuality based on the forecast, modelling of the impacts that rain will have on rivers, the risk of high waves to our coastal communities, and drawing on experience."
He urged people living in low-lying areas to make their own evacuation plans and keep a bag packed with clothing and essential medication for each member of their household.
They should also plan for their pets, Marshall said.
MetService has also issued a warning of coastal swells up to 5m high, which would impact coastal communities.
"The message for tonight is that we have been doing all we can to reduce the impact of flooding on people and property, but people also have to be aware of the risks and take action to keep themselves safe," Marshall said.
MetService meteorologist David Miller said that the area could expect more than half a metre of rainfall.
"Significant impacts can be expected, such as slips, flooding and rapidly rising rivers and streams.
"This event is exceptional not just in its accumulation amounts, but also its prolonged nature, with heavy rain expected to continue in these areas right up until Thursday evening.
Niwa Weather experts say a long-lived atmospheric river will make landfall in the South Island today and will intensify from tomorrow through to Thursday and last throughout the weekend.
The upper North Island and Auckland are now included in the weather warnings, with heavy rain and strong wind expected from Wednesday.
Additionally, a strong wind watch is currently in effect for Northland, Auckland, and Great Barrier Island due to strong north-to-northeasterly winds.
