New Zealand's weather forecast agency MetService will be issuing heat alerts from Wednesday amid an unusually hot summer.

This is the third year that MetService has been trialing alerts to help New Zealanders plan and prepare for extreme heat, the agency said in a statement on Tuesday.

"This year, the influence of El Niño conditions across the Pacific is forecast to bring a hotter than usual summer across the eastern areas of New Zealand leading to additional concern in these regions," it said.

Extreme heat during summer can be draining and have an impact on people's daily lives, especially their health and wellbeing, it said, adding infants, older people, and those with pre-existing medical conditions or on certain medications are more at risk.

International research shows that extreme heat and heat waves can cause illness and death, but effective planning and actions can reduce its effects on health, according to MetService.

MetService meteorologist Georgina Griffiths said heat alerts will be displayed on the MetService website and app for 46 towns and cities.

If a heat alert is triggered it means that people should take precautions, such as keeping hydrated, seeking shade, checking in with vulnerable people and children and looking after their pets and livestock, she said.