Power company Vector is warning customers across the North Island to be prepared for possible power outages, as a forecast storm gets set to hit the country.
Tens of thousands of people are likely to have their planned weekend events dampened by forecast heavy rain and gale-force winds.
Organisers of major weekend events were last night hoping for the best but preparing for the worst as a "nasty storm" bore down on New Zealand.
It is due to make landfall tonight and get more intense as it makes its way over the North Island tomorrow, forecasters say.
Metservice has issued a severe weather warning for most of the North Island and the top of the South Island, with heavy rain expected from Mt Taranaki to Mt Ruapehu, the ranges of eastern Bay of Plenty and Gisborne, Wellington, Wairarapa and the Kaikoura Ranges.
Gales are also forecast from Taranaki and the Waikato across to Bay of Plenty and down to Wellington as well as Buller, Nelson and Marlborough.
WeatherWatch head analyst Philip Duncan said a rapidly deepening low was behind the storm.
"It will go from a non-existent mark on our weather maps today to a nasty storm by Saturday morning - with the bulk of the deepening happening in a 24-hour timeframe."
Mr Duncan said the low had the potential to reach "weather bomb" status - where the central pressure falls 24 millibars in 24 hours.
This was much faster than usual, he said.
Vector is advising customers of the possibilities of outages as the stormy weather hits early tomorrow.
Chief executive Simon Mackenzie said crews have been put on alert and all efforts will be made to respond to outages as soon as possible.
"We do not expect too many issues but recommend that customers always be prepared for the possibility of an outage during inclement weather," he said.
The storm could cause strife at numerous events. Women planning to attend the TV3 Derby Day at Ellerslie Racecourse in Auckland tomorrow might need to make sure their fascinators are fastened securely.
Organisers were expecting 8000 to 10,000 people at the event featuring 10 races, the National Fashions in the Field final and a performance by singer Kimbra.
An Ellerslie spokeswoman said extra marquees had been ordered and the fashion event might be moved indoors.
The weather could also put a dampener on the Black Caps' final one-day international against South Africa, at Eden Park tomorrow.
"At this stage we're hoping that we'll get enough nice weather so we can still get some play in," said Eden Park marketing manager Tracy Morgan.
"All we can do is plan on the basis that we will get some play."
With luck, the Warriors' grand final rematch against the Sea Eagles on Sunday afternoon - the second match of Eden Park's double-header weekend - will be safe from the storm because the skies are predicted to clear that morning.
The Sounds in the Sun event is set to go ahead at Unitec's Mt Albert campus tomorrow - even if it is in the rain. Up to 10,000 people are expected at the day-long concert to hear performers such as Shapeshifter, Katchafire and Kidz in Space.
Event manager Adam Basham said organisers were keeping their fingers crossed that the weather would turn around.
But sometimes rain can really make a gig. "And we've also got activities like a waterslide, so if you're going to get wet, you might as well get really wet and get involved."
* Due to make landfall this evening.
* Gale-force winds in much of the North Island.
* Destructive winds in some places.
* Heavy rain tomorrow.
* Clear skies and dry on Sunday in most regions.
* Big seas and hurricane-force winds offshore in Hawkes Bay and Wairarapa on Sunday.