Metservice meteorologist Tui McInnes said the low-system would invite more humid air and wet weather to much of the country.
"Showers along with a period of rain are expected at times for much of the country", McInnes said.
"Some of these showers are showing potential to be heavy or thundery inland, but this is gazing far into the future, so it's important to stay up to date with our latest weather information."
As a result of wetter and cloudier weather, cooler temperatures were expected mid-week, McInnes said.
But "given it's December, cooler is a relative term, so no need to be grabbing your woollen jacket", he said.
Another high-pressure system is set to return to our shores and push away the unhappy low-pressure system as the week draws to a close.
Monday's weather forecast
Whangarei: Cloudy periods. Southwesterlies. High 23C, Low 15C.
Auckland: Often cloudy, chance shower or two. Fresh southwest easing. 21C, 16C.
Tauranga: Becoming cloudy with showers developing from afternoon. Sea breezes. 26C, 17C.
Hamilton: Fine breaks, a few showers possible in the afternoon. Southwesterly dying out. 25C, 14C.
Napier: Cloud increasing. A few showers and southerlies from early afternoon. 24C, 14C.
New Plymouth: Cloudy periods. Southwest turning southeast in the evening. 24C, 13C.
Wellington: Cloudy. A few showers from late morning, clearing afternoon. Southerlies dying out. 18C, 12C.
Nelson: Fine at first, then partly cloudy. Light winds. 22C, 14C.
Greymouth: Often cloudy with light rain, but longer afternoon fine spells. Southwest breezes dying out. 20C, 13C.
Christchurch: Occasional rain, clearing by midday but remaining cloudy. Southwesterly turning northeast afternoon. 17C, 11C.
Queenstown: Early showers, then increasing fine spells. Cloud increasing again evening. Light winds. 24C, 11C.
Dunedin: Fine apart from a morning shower or two. Cloud increasing evening as southwest breezes tend northeast. 15C, 11C.
Invercargill: Early showers, then increasing fine spells. Southeast breezes. 17C, 9C.