MetService meteorologist Amy Rossiter said the weather watch applied from 4pm today until 1am Wednesday and covered the northeast of the country, as far south as Coromandel.
Northland could expect quick, heavy bursts of rain, while more persistent rain was forecast in the Bay of Plenty, where a weather warning was in place.
The current conditions were caused by a humid northeasterly flow over the country. Warm air carried more moisture than cold air, she said.
At the same time, a low-pressure system was developing over the Tasman Sea with a series of fronts and troughs that would bring the rain.
On Monday afternoon, Kerikeri had 93 per cent humidity and Dargaville 90 per cent. Whangārei recorded a more bearable 83 per cent.
Rossiter said there was no relief in sight from the humidity because moist subtropical air was expected to continue flowing from the north.
MetService was also keeping an eye on a cyclone developing over the Coral Sea, northeast of Australia.
Tropical cyclone Ruby was expected to track past New Zealand but it was too soon to be sure of its path.
The current focus was on the system over the Tasman Sea which would deliver "some pretty severe weather", she said.
Beating the heat
High humidity is uncomfortable because it interferes with the body's normal cooling mechanism.
When we get too hot our roughly 2-4 million sweat glands produce sweat, which cools the body as it evaporates from the skin.
That's because it takes energy to convert liquid water, in this case sweat, into vapour. That energy is absorbed from the skin in the form of heat, hence the cooling effect.
Your dog uses the same principle when it sticks out a wet tongue to expose it to the breeze.
However, if humidity is high, the air is close to saturation and can't absorb much more water vapour, so the body's favourite cooling trick no longer works.
We keep sweating but the sweat can't evaporate and we end up being wet and sticky as well as hot.
The simplest remedy is, of course, to turn on the air conditioning or a dehumidifier.
That's not always an option - and, besides, both consume significant amounts of power.
Other things to try include:
■ Wear light, loose-fitting clothing in natural fabrics such as cotton.
■ Avoid strenuous exercise.
■ Open windows or doors on opposite sides of the house to set up a cooling breeze. If there is no breeze – how often does that happen in Northland? – you could set up a fan by an open window to push warm air out. That will draw in air from the other side of the house.
■ Some people get relief at night by putting their pillow, or even their sheets, in the freezer for a few hours before bed. A frozen hot water bottle does the same trick.
■ Take a cool shower.
■ Take a cool shower, then instead of drying off, stand wet in the draught between open doors or windows on opposite sides of the house. This is highly effective due to the vapourisation principle described above but requires caution, lest your neighbours or passers-by complain about indecent exposure.
