Cyclone Liua is the first cyclone to form in the south-west Pacific in the month of September for nearly 70 years.
Liua is over the Solomons' Makira Island and is forecast to develop into a category two on the scale of one to five.
Its winds are weak, but forecasters are concerned about its slow pace and heavy rains.
The official cyclone season runs from 1 November to the end of April.
Tropical Cyclone #Liua has formed in the Southwest Pacific near the southern Soloman Islands— NIWA Weather (@NiwaWeather) September 26, 2018
This is the first TC to form during the month of September in the SW Pacific since Tropical Cyclone 02P on 12 Sep 1950 or in 68 years! [data: IBTrACS] pic.twitter.com/JRnWBw2hVO
Chris Brandolino of the New Zealand climate and weather office, NIWA, said Liua comes even before forecasters have put together their cyclone outlook for the region.
"It is very unusual, in fact this is the first tropical cyclone to form in the month of September going back to 1950. It is certainly unusual."
Mr Brandolino said the official outlook from regional forecasters should be out next month.
Fiji Meteorological Service's Stephen Meke also said September cyclones were extremely unusual.
But he said the conditions in the whole region were months ahead of usual.
"Though we are still in the transition months, the environment has presented itself and allowed for this TC to fire up. The conditions for tropical cyclones are there for the South Pacific, that's very true."
He said there was a high likelihood of an El Niño developing this summer, which usually brings more intense cyclone seasons.
Tropical Cyclone Fehi was the first to form last season and affected New Caledonia and New Zealand in late January.