"People are advised to keep up to date with the latest forecasts in case this watch is upgraded to a full warning or further areas added,'' MetService said.
"After a surge of cooler air [tomorrow], look out for an even colder one to end the week.
"Subtle differences mean that this second cold snap will likely bring snow to southern and western areas than the first.''
Today's weather is mostly cloudy with a few showers in and around Auckland city. The same is forecast in the Coromandel Peninsula, western Bay of Plenty and Gisborne.
Aucklanders can expect the odd shower north of Whangaparāoa and spreading elsewhere from this afternoon. There is a possibility of heavy rain this evening and northwesterly winds.
A high of 17C is expected in the City of Sails today and an overnight low of 10C. The same high is forecast in Hamilton, with a little colder overnight low of 7C.
There will be showers - possibly heavy and thundery from this afternoon - in Whāngārei. Those conditions are expected to ease by this evening.
Those in Northland will get a mostly cloudy start to the working week; with showers in the far north - some heavy - spreading south and possibly thunderstorms.
The rest of the North Island, as well as Marlborough, will be often cloudy today.
Those in Wellington get a mainly fine day. A high of 15C and a chilly 8C low is forecast.
Further south, it will be a mostly cloudy day in Nelson and Buller, with one or two showers forecast. Rain develops in Buller tonight.
In Canterbury, there is high cloud and a few spots of rain that are expected to clear by dawn.
People in Christchurch can look forward to a fine day with high cloud and a high of 15C. But make sure to wrap up tonight, with an overnight low of 6C expected.
Those in Otago and Southland will see mostly cloud as well as patches of rain.
It will be a cloudy day in Queenstown - save for some rain at times. An overnight low of 2C brings a chilly start to the week. Invercargill is also in for a cold one tonight, with a 3C low forecast and a high of 11C today.
In the Chatham Islands, the odd shower and cloudy conditions are expected.
