Although summer does not officially start until the beginning of December, Niwa National Climate Centre forecasting principal scientist Chris Brandolino said New Zealand was now on La Nina alert and temperatures were "very likely" to be above average right the way across the country.
There would also be a period of particularly warm conditions from next week, when temperatures could reach 25degC in some parts of the South.
Coastal sea surface temperatures (SSTs) ranged from 0.5degC to 1.0degC above average during October, and they were predicted to become more unusually warm over the coming month or two, he said.
This could culminate in marine heatwave conditions in some regions.
Mr Brandolino said higher-than-normal air pressure was favoured over the South Island and to the south and east of the country, causing more easterly quarter winds which increased the chance for dry spells in the South Island and western North Island in particular.
"The development of La Nina-like patterns may contribute to drier conditions in the coming months around the hydro lake areas."
Rainfall was most likely to be below normal in the west of the South Island, and about equally likely to be near normal or below normal in the south and east of the island.
Soil moisture levels and river flows were about equally likely to be near normal or below normal across the southern regions, he said.
