There would be more certainty in the next 72 hours of its exact tracking.
Heavy rain and strong winds could hit Rotuma in the north as early as Boxing Day, and as the system moved closer to Fiji, there would likely be damaging to destructive-force winds and heavy rain which could lead to flooding.
READ MORE:
• Climate change: How hard will extreme weather hit NZ?
• Climate change: Revealing its hand in today's extreme weather
• Extreme heat, disease and rising seas: how climate change threatens Auckland
Storm surges could also be expected along the coast.
Those out at sea could expect strong winds with rough waters from Wednesday, with conditions deteriorating further later in the week.
Members of the public are advised to remain alert and vigilant, and take warnings and advisories.
If it forms it will be named Tropical Cyclone Sarai.
It will be the second tropical cyclone of the season - which runs from November to April - after Tropical Cyclone Rita formed at the end of November.
New Zealand MetService meteorologist Michael Martens said its weather models were also indicating a "heightened risk" of a tropical cyclone developing northeast of Vanuatu, just west of Tuvalu, by the end of the week.
People planning a trip there should keep up to date with the forecast, he said.
At this point there was no concern for New Zealand, but MetService would be monitoring the system closely.
Normally about 10 tropical cyclones form in the Southwest Pacific Basin over the season.
Niwa and MetService predict about nine to 12 cyclones will form, with elevated activity late in the season, from February to April.
Tropical cyclones are categorised in strength from 1 to 5, with 5 being the most intense.
For the coming season, about four storms are expected to reach at least Category 3 strength, with mean wind speeds of at least 118 km/h.
Past years with conditions similar to this one suggest several storms that develop could intensify to at least Category 3 strength.
Category 5-strength cyclones, where wind gusts exceed 199 km/h, have occurred in some years with similar conditions leading into the 2019/20 season.
Cyclones are essentially low-pressure systems that form and build over warm waters in the tropics - but with extreme characteristics.
Gale-force winds - or those higher than 63km/h - are found at low levels near their swirling centres but can fan out for hundreds of kilometres.
Meteorologists consider a "severe tropical cyclone" to be one that blasts as hard as 118km/h.
At least one comes within 550km of New Zealand each year, usually around February and March.
To get this far south, they have to make their way over much colder waters, while hitting strong upper-level winds as they move out of the tropics.
By the time they arrive, they are almost always reclassified as "ex-tropical" cyclones.
That does not mean they have weakened or been downgraded, but rather that they've morphed into a completely different beast.
And ex-tropical cyclones still pack the potential for severe weather.
Under the right conditions, they can intensify and even muster lower pressures than they had before being reclassified.
