While Yasa was likely to cause damage and possible evacuations in Fiji, by the time it reached New Zealand, Duncan said it might not even be a storm.
"Every single tropical cyclone loses its warm core just as it approaches New Zealand. Some, like Cyclone Gita a couple years ago, can retain the look and power of a cyclone during this process. Others fall apart.
"Modelling certainly is hinting that any possible New Zealand hit may not be from a storm at all - but simply the remnants of an ex-cyclone, in the form of a low-pressure system and rain."
Given Yasa's volatility, Duncan said there were currently a few different scenarios for Kiwis on Christmas Day.
"High pressure dominates, Yasa's remnants fail to arrive, dry for most. The remnants of Yasa fall apart over New Zealand bringing rain and showers. A combination of the above with high pressure bringing dry areas to some regions, but Yasa's remnants bringing some cloud and wet weather too."
By the time Yasa reached New Zealand, there was only "a low risk this may be a 'storm' still by then in our part of the world".
However, its impact on Fiji looked concerning.
It was expected to "have a decent impact on parts of Fiji", possibly bringing some wind, rain and dangerous seas around Vanuatu for a time from the east.
Data indicated Yasa could grow into a severe category 4 cyclone or stronger.
"Currently it's spinning in roughly the same spot out at sea. It's likely to remain in this similar location across Tuesday and Wednesday.
"However, from late Wednesday and across Thursday and maybe Friday, Cyclone Yasa could have a major impact on Fiji as it starts to move southwards."
That could see serious flooding, slips and structural damage and could see Fijians having to evacuate to higher ground.
"This now more direct path into Fiji is of serious concern, especially with Yasa projected to be a major cyclone strength-wise by then ... potentially category 4 which can have sustained winds of almost 200km/h."