"While those thunderstorms were dramatic, they were actually fairly isolated - the only difference was they were isolated over main cities.
"This system that's coming in tomorrow and Wednesday is much bigger. It affects a much larger area; but in saying that, we don't know if this severe weather will hit those main centres or more rural areas.
"It's a very big system, but sometimes when they're big it means the energy is spread over a much greater area."
Mr Duncan said the weather was typical for spring, although it was a little unusual to get a tropical storm, rather than a Southern Ocean storm, at this time of year.
MetService meteorologist John Law said this week's low would move in from the north and slowly push towards the east, reaching the top of the South Island late tomorrow.
"We are expecting some strong winds across the North Island, even gales across much of the area, and some very heavy rainfall, so I think it's a much broader area affected than we saw at the weekend."
MetService has issued heavy rain and severe gale warnings for Northland, Auckland, the Coromandel Peninsula and Gisborne from tomorrow and early Wednesday.
The heaviest rain was likely in Gisborne, where up to 150mm was expected in the ranges by early Wednesday, and eastern Northland and Coromandel, where 100mm was expected.
Streams and rivers would rise rapidly, with slips and localised surface flooding also likely.
The strongest gales were expected in Northland, Auckland and Coromandel, where severe southeasterlies would hit from tomorrow afternoon.
MetService said damaging gusts of up to 130km/h were likely to bring down trees and powerlines and make driving dangerous in Auckland and Coromandel.
The low was expected to gradually weaken on Thursday, bringing heavy rain across eastern parts of the South Island before departing by the end of the week. However, disturbed westerlies were likely to take its place.
