New research shows the venomous Australian redback spider has mated with New Zealand's native katipo to produce a stronger hybrid species.
The interbreeding could also eventually cause the extinction of the katipo, which is also poisonous.
The redback spider is already established in Central Otago and New Plymouth and the AgResearch study suggests it could easily survive and multiply from Northland to Christchurch.
AgResearch scientist Cor Vink says the hybrid redback could spread into areas normally unsuitable for it to live.
Dr Vink says he has seen the result of a redback mating with a katipo in Gisborne, producing a more versatile and larger than normal male species than can live in urban areas.
The male hybrid spider was almost the size of a female redback.
Dr Vink says it was originally thought the redback could not survive in even moderately wet areas of New Zealand, but the research shows it could thrive in warm, dry parts of many eastern regions - as long as it has shelter from the rain.
The spread of the redback is likely to happen as the result of new invasions, rather than from existing populations, Dr Vink says, and its real threat is to the conservation of native species.
Wellington pest control company Target Pest says all spiders are difficult to eradicate and dealing with an urban redback-katipo hybrid outbreak would be no different.