The MetService reported yesterday that the deep low-pressure system could bring more than triple the agency's warning criteria level of rainfall.
A large front the MetService called a "combination of significant weather features" with "explosive cyclogenesis" was forecast to move onto the lower South Island late today, then cross much of the South Island tomorrow, and the rest of the country on Thursday.
A MetService spokeswoman said this morning more would be known when fresh models were checked about midday.
The forecaster reported yesterday that each of the fronts was expected to bring heavy rain to western areas and strong winds in the east.
For Fiordland and Westland, there was moderate confidence that heavy rain warnings would be required today.
For Fiordland, Westland, Buller, northwest Nelson and the Tararua Range, there was a high level of confidence that rainfall accumulations would meet warning criteria tomorrow and Thursday.
For Southland and Clutha, there was a moderate confidence of rainfall reaching warning criteria on Thursday.
"Our warning criteria is 100mm of rainfall in 24 hours, but some of these ranges and more exposed area could get 300mm in 24 hours," MetService forecaster Lisa Murray said.
"So this is a really significant event rain-wise because of those accumulations and there is probably going to be widespread flooding at times, slips and rivers rising rapidly."
Although Kiwis understood how severe these systems could be, Murray said tourists in affected areas like Fiordland and along the South Island's West Coast needed to be aware of what was coming.
"The tourists don't really understand that when we get a deluge of rain like this, just what impacts it can have on the country and on the microclimates within New Zealand."
It wasn't just hard rain on the cards - strong winds could exceed wind speeds of 120km/h.
Strong to gale northwest winds were likely to affect eastern areas of the South Island and southern North Island for extended periods later in the week.
For Wairarapa, Wellington and the east of the South Island, there was a high chance of severe northwest gales tomorrow and Thursday.
"We are talking about damaging winds here. We are talking, tie down the trampoline," Murray said.
"Most of the South Island will be affected, especially high country areas."
For coastal Southland and Stewart Island, there was moderate confidence that westerly winds would rise to severe gale at times today.
By the end of the week, the dominant weather over New Zealand would return to a southwesterly flow.
Main centres: highs and lows
Auckland: Today: 23C/16C Tomorrow: 24C/16C Thurs: 22C/14C Fri: 21C/13C
Hamilton: Today: 25C/12C Tomorrow: 25C/15C Thurs: 22C/10C Fri: 21C/8C
Tauranga: Today: 28C/16C Tomorrow: 27C/17C Thurs: 23C/12C Fri: 23C/12C
Wellington: Today: 20C/16C Tomorrow: 19C/15C Thurs: 19C/10C Fri: 18C/11C
Christchurch: Today: 29C/13C Tomorrow: 26C/12C Thurs: 21C/7C Fri: 17C/9C
Dunedin: Today: 19C/11C Tomorrow: 17C/9C Thurs: 16C/8C Fri: 15C/9C
