But Wellington is set to buck the trend, with long fine spells forecast. "Wellington should have quite a nice day tomorrow," Mercer said.
It was the same with the Wairarapa.
However Weatherwatch.co.nz warned the positioning of the low meant even a small change could result in wet weather for the capital.
Despite the wet, the weather should stay reasonably warm, with Auckland, Northland and Gisborne all reaching the high teens or even 20C.
The rest of the week is also set to be a mixed bag, with an active front crossing the country on Wednesday and Thursday bringing rain to large parts of New Zealand.
Friday looks settled but there is uncertainty towards the end of the week.
High cloud and rain have already marred Labour Weekend for much of the country, and the weather could spell more trouble for holidaymakers heading back from the Coromandel and Bay of Plenty today.
The road toll already stands at two for this Labour Weekend - down from three last year.
Just after midnight on Saturday, Christchurch woman Jaimey Leigh Fellows, 32, was killed in a car crash near Burnham Military Camp. And a female motorcyclist died in a crash on SH41 near Turangi later on Saturday morning.
LABOUR DAY WEATHER
WHANGAREI
Rain in morning, some heavy. Thunderstorms possible in afternoon. Northerlies.
High 20C, Overnight 12C
AUCKLAND
Rain developing in morning, possibly heavy and thundery in afternoon, clearing evening. Strong northerlies. High 19C, Overnight 12C
HAMILTON
Rain developing morning, chance heavy and thundery afternoon, clearing evening. Northerlies. High 17C, Overnight 8C
TAURANGA
Rain developing morning, heavy and possibly thundery falls from afternoon, easing. Northerlies. High 18C, Overnight 11C
NAPIER
Occasional rain developing in the morning, clearing evening. Northwesterlies.
High 18C, Overnight 9C
WELLINGTON
Partly cloudy, long afternoon fine spells. Northwesterlies.
High 15C, Overnight 11C