It has also promoted warnings about favourable conditions for wildfires.
MetService said a warm, moist northwesterly flow is sending rain to the west and south of the South Island/ Te Waipounamu, prompting a heavy rain watch for southern Westland until Thursday night.
But the flow is heating up and drying out as it travels over the Southern Alps, raising the mercury in the east with Christchurch and Dunedin reaching 31C this afternoon, Oamaru 3C and Timaru 33C.
In the north, settled and sunny days continue into Friday with above average temperatures and mostly isolated showers.
MetService meteorologist Clare O'Connor said a weak cold front would travel up the South Island overnight on Friday, bringing easterly winds and low cloud, but it would be short-lived and warmer temperatures would return before a cooler change on Monday.
Heavy rain might reappear on the south and west of the South Island again from Saturday afternoon, with temperatures expected to rise again in the east.
That would include the east of the North Island with predictions Hastings could reach 33C on Sunday.
Heat alerts would be issued for areas reaching their highest temperatures and MetService advised people to remain sun smart and keep hydrated.
Parents urged to take care with young children in heat
Te Whatu Ora/Health New Zealand has issued a reminder to parents to ensure their babies and toddlers were kept hydrated and did not get too much sun.
The National Institute of Water and Atmospheric Research (NIWA) was predicting temperatures could rise above 35C in some areas.
Starship Children's Health paediatrician Dr Jin Russell said babies were less capable of regulating their temperature and could quickly become overheated or dehydrated in hot weather.
"Most importantly - never leave your baby or toddler alone in the car - a car's interior can heat up very quickly when the windows are wound up and your child will overheat very quickly which can be potentially fatal," she said.
"Toddlers also need some extra care when it's hot too. Offer them plenty of water to drink, make sure they're wearing a hat, and cover up in the sun or let them play in the shade. Remember to slip, slop, slap and choose a 30+ broad spectrum hypoallergenic sunscreen that is suitable for young children.
"Playing with water in a shady spot outside is a great way to cool down. Always supervise toddlers in or near water. Home-made ice blocks made with fruit juice are an excellent and healthy way to help your child cool down."
Anyone worried about their child can call Healthline for free health advice.
Fire ban issued for parts of Canterbury
A total ban on outdoor fires will be in place from Friday morning for mid and south Canterbury's hill and high country.
The shift to a prohibited fire season starts at 8am for Hakataramea Valley and the Rangitata, Ashburton and Rakaia Gorge areas.
It means all fire permits were suspended.
Fire and Emergency district manager Rob Hands said the tussock grasslands would continue to dry out over the next few weeks, creating conditions where wildfires start easily and could quickly get out of control.
Last month, a controlled burn escaped near Hakataramea, requiring 10 fire trucks and tankers and three helicopters to put it out.
People should check the conditions before starting any spark-generating activities outside, Hands said.
Water restrictions for Wellington
Much of the Wellington region will move to level two water restrictions on Wednesday at midnight.
The ban on all sprinklers and irrigation systems for residential homes will apply to Wellington, Porirua, Hutt, and Upper Hutt cities.
People can still water their gardens with a hose at anytime as long as it is not left unattended.
