It's likely to pass over Auckland on Saturday and Sunday morning.
Mr Duncan said while it would no longer be classified as a tropical cyclone by the time it reached New Zealand, it would still "pack a punch".
"We expect it will move in around Saturday or Sunday and it could bring very heavy rain and severe gales to northern New Zealand and across the weekend it will affect all of the North Island at some point," Mr Duncan said.
However, it is not yet known which part of the country will be worst hit by the tropical storm.
MetService meteorologist Dan Corbett agreed, saying it was looking like a "batten down the hatches" weekend, which could get quite nasty in some places.
"Certainly northern New Zealand, so Northland to Auckland to the Bay of Plenty, through Saturday, it will certainly be spells of heavy rain, strong winds, gales - maybe even severe gales."
Storm photos from December 2012
Both Mr Duncan and Mr Corbett said the picture of where the storm was likely to go will get clearer as the week goes on and were hopeful that by tomorrow they would have a better idea of where the centre would strike.
Tropical Cyclone Lusi is a category one cyclone sitting near Vanuatu, and is slowly intensifying in the very warm, bath-like waters in the Pacific which fuels such storms, said Mr Corbett.
It is likely to grow into a category three cyclone within 24 hours, he said, and track to the south and west of Fiji.
By the weekend it will reach New Zealand waters.
"It will be weakening as it comes towards us, but it will have already become quite strong," Mr Corbett said.
Additional reporting by APNZ
