Heavy rain is also expected to soak the lower North Island tomorrow.
The Wellington Region Emergency Management Office said downpours are expected to drench the Tararua Range from 9am.
Between 60 and 70mm of rain could accumulate in the following 6-9 hours.
A severe weather warning is also in place for Fiordland, Westland, Southern Lakes, Canterbury high country, Nelson, Buller, Wairarapa, Manawatu and Taranaki tomorrow.
The MetService said the heaviest rain is expected over Fiordland and Westland, the headwaters of Otago and Canterbury, the ranges of Buller and Northwest Nelson.