Forecasting principal scientist Chris Brandolino said La Nina had restrengthened during August, and was expected to be an important climate driver for the whole country over the next three months.
In Southland, inland Otago, the West Coast and the Alps and foothills, temperatures were very likely to be above average, he said.
"More northeasterly winds may cause periods of increased sunshine and warmer temperatures."
He said the season may feature "a battleground of pressure features" in the region, with occasional sub-tropical lows and strong high-pressure belts, which would make seasonal rainfall predictions "particularly challenging".
"Rainfall totals are about equally likely to be near normal or below normal, and fewer westerly winds during spring may lead to lengthy dry spells.
"However, this may be interspersed with occasionally heavy, moisture-laden fronts."
Soil moisture levels and river flows were most likely to be near normal, he said.
In coastal Otago, temperatures were also most likely to be above average, but fewer westerly winds may lead to fewer warm days.
More frequent onshore winds may contribute to more cloud cover and warmer overnight temperatures, he said.
Like the rest of the southern region, rainfall totals were most likely to be near normal with longer dry spells, interspersed with occasional subtropical lows that transported moisture from the north.
Soil moisture levels and river flows in coastal Otago were also most likely to be near normal.
Mr Brandolino said coastal sea surface temperatures (SSTs) ranged from 0.5degC to 1degC above average during August, and marine heatwave conditions were occurring offshore in many regions.
"Of note, many climate models show an increase in SST anomalies (difference from average) during the October-November period.
"Given the high-impact nature of recent summer marine heatwaves, the maritime sector should monitor this situation closely."
La Nina conditions were likely to grow stronger by late spring, and during December-February, there was a 55%-60% chance La Nina would continue, he said.
"A triple-dip La Nina — three consecutive La Nina events from 2020-2022 — is very likely, the first since 1998-2000."
