"From 9pm this evening through to 9am tomorrow morning, they could see 40 to 60 millimetres of rainfall in that time [in Canterbury]."
WeatherWatch.co.nz is also forecasting 30 to 70mm of rain mainly between Dunedin and Christchurch in the next 24 hours.
Christchurch is on the cusp of the heaviest rain but surface flooding in the city is also possible.
This was especially since the quakes which appeared to have altered and slowed down how well rainwater drained away, a WeatherWatch statement said.
The heavy rain watches will end on Sunday at 9am in Otago and 11am in Canterbury.
The upper North Island will have showers passing through for the rest of the weekend, although on Sunday those showers will predominantly be in the west.
"In the North Island tomorrow, we could still be seeing some scattered rain in some western areas, although for the likes of Auckland you shouldn't be seeing anything in terms of showers until the evening, and even then those are further west," Nickson said.
"There could be some cloud around, and then you'll probably get some fine spells, with the chance you'll get a shower in the evening in Auckland."
The upper North Island also has patchy cloud today called "open cell cumulus", which brings showery weather.
The sunniest places to be tomorrow will be Napier and Gisborne.
For the first day back at school on Monday the weather will have cleared slightly across the country, with showers in the west of the North Island, and sun in the Bay of Plenty.
In the South Island on Monday there could be a little bit of cloud around the West Coast but otherwise it's looking mostly fine.
"There could be some low cloud and drizzle around to start the day in the east of the South Island on Monday but that should be clearing and gradually lifting," Nickson said.
"This will become fine for a time in the afternoon but we could get the low cloud returning about the coast in the evening."
Forecast for the weekend
Whangarei
Today: High of 16C. Fine spells, but the odd shower too. Westerlies.
Sunday: High of 15C. Cloudy periods. Chance shower from evening. Southwesterlies.
Monday: High of 16C. Cloudy periods, chance morning shower. Southwesterlies.
Auckland
Today: High of 15C. Cloudy periods. A few showers, some possibly heavy. Westerlies.
Sunday: High of 15C. Cloudy periods, possible morning fog. Few showers developing evening in the west. Southwesterly breezes.
Monday: High of 15C. Cloudy periods, with a few morning showers in the west. Southerlies.
Hamilton
Today: High of 15C. Cloudy spells with a few showers, some possibly heavy. Westerlies.
Sunday: High of 14C. Morning fog or possible drizzle, then fine spells. Chance evening shower. Light winds.
Monday: High of 16C. Morning cloud with chance shower, then increasing fine spells. Light southerlies.
Tauranga
Today: High of 15C. Fine spells, but chance of a shower. Westerlies.
Sunday: High of 14C. Cloudy periods, with longer afternoon fine breaks. Light winds.
Monday: High of 16C. Fine, apart from morning cloud. Light winds.
New Plymouth
Today: High of 14C. Cloudy periods with a few showers, some possibly heavy. Northwesterlies.
Sunday: High of 14C. Showers, possibly heavy, easing at night. Northerlies.
Monday: High of 15C. A few showers gradually clearing, but cloudy periods remain. Southeasterlies developing.
Palmerston North
Today: High of 13C. Cloudy periods and a few showers. Northwesterlies.
Sunday: High of 15C. A few showers developing morning. Northwesterlies turning northeasterly towards evening.
Monday: High of 15C. Morning cloud with chance of a shower, then increasing fine spells. Light winds.
Wellington
Today: High of 12C. Patchy rain clearing this evening as southerlies turn northerly.
Sunday: High of 12C. Cloudy periods. Scattered showers developing morning. Northerlies, turning southerly at night.
Monday: High of 13C. Cloudy periods, with isolated morning showers. Southeasterlies.
Christchurch
Today: High of 10C. Rain setting in this evening. Southerlies strengthening tonight.
Sunday: High of 15C. Occasional rain, easing in the evening. Southerlies.
Monday: High of 13C. Low cloud, chance drizzle, but afternoon sunny spells. Light winds.
Queenstown
Today: High of 9C. Occasional rain. Southerlies.
Sunday: High of 11C. Mostly cloudy, some morning rain. Light southerlies.
Monday: High of 11C. Cloudy periods. Light winds.
Dunedin
Today: High of 11C. Periods of rain. Southerlies.
Sunday: High of 11C. Rain, chance heavy in the morning, easing by evening. Easterlies.
Monday: High of 13C. Low cloud, chance of drizzle, but afternoon sunny spells. Northeasterlies.
