The report - commissioned by Greenpeace NZ - also shows that in most scenarios oil would not make landfall on the Otago coast, because sea currents and climatic conditions would push the oil eastwards.
The report, embargoed until today, comes after oil and gas exploration and production company Anadarko announced almost two months ago it plans to drill a $US100 million test well 60km off the coast of Otago Peninsula, at the southern margin of the Canterbury Basin.
Anadarko has previously said it was more likely to find gas than oil.
The report by Wellington-based data scientist Dumpark shows the potential spread of a major deep-sea oil spill at the two locations - in the Taranaki and Canterbury basins - Anadarko is scheduled to test drill this summer.
The report was based on 1000 oil spill scenarios - ranging up to the scale of the 2010 Deep Horizon disaster in the Gulf of Mexico and smaller - using 10 years of global archives of weather data.
It showed oil would reach the shore of Taiaroa Head in 27% of spills modelled in both summer and winter, and Oamaru in 31% of spills in summer and 23% in winter.
This was in contrast to a high probability of ''dramatic consequences'' in the event of a deep-sea blowout in the North Island. Oil would reach the shore in all of the winter scenarios modelled in most North Island locations.
The modelling showed oceanic currents and winds meant oil was much more likely to wash up on the Chatham Islands than the Otago coast.
The Otago Daily Times will today seek comment from Anadarko on the embargoed Dumpark report.
Dumpark data analyst Timo Franz said even if oil did not wash up on the Otago coastline, it could still affect fisheries off the coast and feeding grounds for marine birds.
Greenpeace campaigner Steve Abel said the report showed the risks of oil drilling.
''This modelling suggests just how much of a threat deep-sea drilling could be to our values, our seas, our beaches, our way of life and our economic prosperity,'' Mr Abel said.
In the worst-case scenario for Taiaroa Head, oil would first wash ashore in four days and in a worst-case scenario for Oamaru, in three days.
A week after a worst-case scenario spill, there would be 14.9 grams of oil per square metre of water at Taiaroa Head, and after six months there would be 23.8g/m2.
This was above the 0.01g/m2 threshold which resulted in the closure of fisheries, the 1g/m2 which would require shoreline clean-ups and 10g/m2 threshold which meant sea birds and other wildlife could be killed.
A week after the worst case scenario for Oamaru, there would be 2.7g/m2 and after six months there would be 16.2g/m2.
