Humidity, heat, heavy rain and thunderstorms this week are a "taster" of the developing La Niña conditions expected to influence our weather in coming months.
La Niña is a complex weather system caused by strong wind blowing warm water at the ocean's surface from South America and across the Pacific Ocean.
This tends to bring more northerly winds and has historically been associated with warm Tasman Sea temperatures.
It also often leads to warm, moist and rainy conditions for northern and eastern areas - something we're experiencing at the moment.
This week's warmth and humidity is being caused by air flow as moisture is transported southward from the tropics.
"Warm northerlies will continue through Thursday, transporting some moisture southward from the tropics in the process," NIWA says.
The "unseasonable warmth" will continue into the weekend - while NIWA warns some heavy rain and thunderstorms will be possible on Thursday-Friday.
MetService is forecasting a return to more "spring-like conditions" as the working week ends.
"On Thursday, the front and associated low pressure to the west of Aotearoa moves over the country to bring rain to most western regions," MetService says.
"The North Island and in particular the Bay of Plenty region will see a risk of the heaviest rain.
"On Friday, this low pressure and front continues to slowly move eastwards bringing more rain to the North Island, and the top of the South Island as edge closer to the weekend."
