Head of weather news Heather Keats said much of New Zealand would be affected, and several "significant" orange warnings for both heavy rain and northwest winds were issued on Sunday.
"We're talking gusts up to 130km/h for the Canterbury high country and the foothills of the Canterbury Plains, as well as Wellington, Wairarapa, and the Tararua district," she said.
"120km/h for Otago, for the deep South, and for Marlborough.
"The heavy rain is expected to be the most significant for the western side of the alps, as per usual; however, there will be spillover into the headwaters of the Canterbury and Otago lakes and rivers."
Heavy rain warnings stretch from Fiordland to parts of Marlborough.
But that was not the worst of it, as "an even more impactful weather system" was expected to arrive on Thursday, Keats said.
There would be more warnings issued closer to the time, she said.
"The potential is there for this to be a highly impactful event."
"Gale northwesterlies tied in with a very significant front and a deep low pressure system to the far south of the country which will bring cold southwesterlies. Thursday will be a big day in the South and for central New Zealand."
"On Friday, that front will move over the North Island, it is weakening, but still some wet and windy weather to track with it," Keats said.
The Queenstown Lakes District Council this morning said heavy rain was falling across the district, and was due to continue for most of the day.
"If we get the level of rain forecast, there's a good chance we'll see some surface flooding, particularly on roads near waterways."
The council urged people to take care on roads and to keep an eye on its social media posts for updates on the situation.
Keats said the country was experiencing was typical spring weather.
Because there was little reprieve between the fast-moving fronts, it was feeling like a "very active" spring.
"Yes, it will calm down, but it doesn't look likely this week and maybe even to next week as we see again those fast fronts as they cross over us," Keats said.
- additional reporting Allied Media
