New Zealanders are being urged to stay inside, drink lots of water and limit alcohol this week as temperatures are set to soar by about 6C above average.

The mercury neared 35C in Blenheim yesterday as the heat from across the Tasman began affecting New Zealand.

MetService says the hottest temperatures over the week will likely be felt in the eastern parts of the country, with Tīmaru and Blenheim to Napier, and Hastings expected to simmer in heat up to six degrees higher than average.

MetService says the heat had been blown in from Australia and was expected to hang around for the next five days.

Blenheim is expected to reach 35C today, with Gisborne close behind on 34C.

Kerikeri, Paihia, Tauranga, Napier, Hastings, Masterton, Nelson, Christchurch, Ashburton, Wanaka and Alexandra were all expected to top 30C, with other centres largely in the high 20s.

Canterbury District Health Board medical officer of health Alistair Humphrey says people needed to get ready, which meant drinking more water, avoiding alcohol and finding shady spots indoors to keep cool.

He says it was also important to keep a close eye on children and the elderly, who were more vulnerable to the harsh conditions.

"Look out for your friends and the people you work with, particularly children and particularly the elderly people because the often don't know when they're getting dry."

Doctor Humphrey says people needed to remind their friends how important it was to slip, slop, slap and wrap.

Fire and Emergency says the fire danger was particularly bad in Northland, Nelson, Marlborough and Canterbury where there are total fire bans in place.

Principal rural fire officer Thomas Harre says people in those areas needed to be especially careful, which included not mowing lawns during the hottest part of the day, which was likely to be from midday up until 5pm.

People should also check their gas bottle connections, he says.

"But the biggest one would be just not to have fires, and go to the checkitsalright.nz website which has a whole list of information around fire seasons and fire types and conditions around it."

Thomas says in the rest of the country there is a restricted fire season which means that people need a permit from Fire and Emergency New Zealand to have a fire.