The most significant rainfall in 10 years is predicted to fall in the Canterbury region this weekend.
MetService has issued a severe weather warning for Canterbury for the weekend, with 200 to 300mm of rain forecast for the high country, and 80 to 120mm about the plains and the coast.
North Canterbury and the Kaikōura coast were expected to be the worst affected, with other parts of Canterbury also in for wet and windy conditions.
The heavy rain will start on Saturday evening and continue until late Monday.
Weather commentator Richard Green told Checkpoint he would put the predicted weather in the "severe category".
"It is very rare that Canterbury has a ... red alert status from MetService. It's the terminology ... things like 'dangerous river conditions and flooding are expected', 'slips and floodwaters are likely to disrupt travel for Cantabrians, some roads impassable' and 'possibly floodwaters isolating communities'.
"That's terminology I've never come across from MetService. We don't want to push the panic button here, but to give you an idea how heavy this rain is looking, it's starting mid-afternoon, late afternoon tomorrow and it's falling on very dry ground, we're in drought territory here and the amount of rainfall ... in the alps is not unusual.
"However, it's what's falling across the plains, coast and maybe Christchurch - 80 to 120mm, now, this year so far we have had for some parts of north Canterbury, 30mm. We're looking at trebling that or quadrupling that in 48 hours."
Canterbury: significant rain event likely from Saturday-Monday— NIWA Weather (@NiwaWeather) May 27, 2021
Fueled by an atmospheric river of moisture
⚠️ With very heavy rain falling upon cracked, dry soils, flooding is a possibility
Map: risk for rain to exceed
10 | 25 | 50 | 100 mm
Some | Medium | High pic.twitter.com/SES6tXJTIZ
He advised keeping an eye on the Canterbury rivers and said farmers would be preparing by moving stock to higher ground.
"This is significant, it's not to be sneezed at... this is first red alert for Canterbury that I have seen from MetService."
He pointed out that the alert was for 44 hours.
"If it does go on for that long, there could be significant problems."
What the authorities are saying
Waka Kotahi - the New Zealand Transport Agency - senior manager maintenance and operations Wayne Oldfield said the rain had potential to cause widespread flooding, land instability and for some rivers to break flood banks.
⚠Weekend Severe Weather ⚠— MetService (@MetService) May 27, 2021
Complex low brings periods of heavy rain, persistent with significant accumulations in Canterbury. Possible severe SE gales for the West Coast.
Read all the details, and stay up to date at https://t.co/qHyE5zhh6X
Christchurch City Council said the city might escape some of the worst of the rain, but with king tides also expected, measures would be put in place to reduce the flood risk.
Contractors were clearing grates and making checks on the stormwater network, the council said.
People in Canterbury were being encouraged to clear out drains and gutters before the heavy rain arrived.
The Fire Service recommended that if people had not cleaned out their spouting, gutters and drains recently, now would be a good time to do it.
It also said that if water leaked through light sockets, to turn off the power as short circuits could cause fires.
Heavy rain is also possible around Mount Taranaki, the Tasman/Nelson ranges, and the Bay of Plenty region east of Whakatane.