The latest run of weather forecasting computer models are now showing the upcoming weather event will also slam the upper South Island too.

WeatherWatch.co.nz says strong winds are likely to affect the northern half of the South Island with heavy rain quite likely moving in to Canterbury on Tuesday and Wednesday.

The low has the potential to bring damaging winds to areas that receive the strongest easterlies, this being eastern Northland, western Coromandel, eastern Waikato, parts of Taupo, parts of Taranaki, eastern Manawatu, Horowhenua, Wellington, southern Wairarapa, Marlborough and the West Coast.

Heavy rain is expected to be widespread across New Zealand with 100+mm likely for some low lying areas.

In northern regions, such as Northland and Auckland, this could cause flooding after recent heavy rains.  Also, with Autumn leaves still slowly falling from trees, it largely increases the risk of surface flooding in Auckland.

Flooding is also possible around Nelson and Canterbury.

WeatherWatch.co.nz says these forecasts are based on the latest three sweeps of computer models and once the low has formed forecasters will have a clearer idea as to who will be hit hardest.

The computer models have been picking this storm since last weekend.

WeatherWatch.co.nz advises all New Zealanders, particularly from Christchurch northwards, to be well aware of the approaching system which is likely to have a significant impact on some parts of the country, starting Sunday and lasting through until Wednesday.