Orange heavy rain warnings are in place for both districts, as well as the headwaters of the Canterbury lakes and rivers, north of the Hurunui River.
Flooding and slips have occurred on some highways in the area.
State Highway 7, between Springs Junction and Reefton, is shut due to flooding and the risk of slips, and SH67 was closed at Mokihinui for a time owing to a large slip. It has since reopened.
People are being urged to avoid non-essential travel as water in the Grey District backcountry moves towards the coast.
The Ahaura and Waipuna catchments saw heavy rain on Friday morning and by 11.30am the Ahaura River had started to plateau, and the Waipuna was falling.
The Grey Emergency Operations Centre was activated at 11am.
"If you are in a flood-prone area and feel uncertain, you may need to self-evacuate," the Grey District Council said.
"Atarau Road was being watched closely this morning but rivers were starting to fall."
Awahono School at Ahaura sent children home. West Coast Emergency Management group manager Claire Brown said as a precaution, schools were being contacted as water levels rose at Coal Creek and Kaiata, ahead of the school bus runs.
NZ Transport Agency (NZTA) has warned motorists to take extra care between Lewis Pass and Springs Junction, between Reefton and Ikamatua, and between Arthur's Pass Village and Kumara Junction.
There have also been rockfalls on State Highway 6 near Meybille Bay.
A front moving up the South Island is set to stall over on Friday, embedding unsettled weather over the upper South Island and lower North Island.
That has seen heavy rain warnings for the Buller and Tararua ranges until Sunday and a strong wind watch extending for almost two days for the bottom of the North Island.
The 42-hour wind warning covers Wellington, Wairarapa and the Tararua District until 8pm on Sunday, and could see northwest winds approaching severe gale in exposed places.
MetService said there was a moderate chance the watch could be upgraded to a warning.
The capital can expect cloud with occasional drizzle and winds gusting up to 100kmh.
Gusty northwesterlies will bring summery temperatures to parts of the eastern North Island with 28C expected in Hastings and 27C in Napier.
A strong wind watch is also in place for Marlborough until 2pm on Friday.
Rain will ease briefly for the West Coast on Saturday, before another front arrives from the south later in the day, bringing further showers.
Nelson Lakes and the Tasman District west of Takaka have heavy rain watches in place until mid-afternoon and midnight Saturday respectively - both have moderate chances of being upgraded to warnings, MetService said.
The east coast of the South Island is in for a fine start to the weekend with temperatures between 21C and 24C expected from Oamaru to Culverden on Friday.
