The spider was cautiously identified as a Australian invader yesterday morning by Wanaka Department of Conservation biodiversity ranger Florence Gaud.
"People live with them in Australia. If you leave them alone, there shouldn't be any issues with them," she said.
The live spider has been couriered by the Otago Daily Times to AgResearch scientist Cor Vink, at Lincoln.
Mr Barrow said he read yesterday's Otago Daily Times "Ask A Scientist" column, by Simon Pollard, about the native katipo spider, and believed his spider was one of those.
The katipo is also poisonous and has a pattern on its back similar to that of a redback.
Spiders didn't usually faze him but knowing his one was probably an Australian redback "gives me the creeps", he said.
Mr Barrow was not bitten when he washed the spider out of silver beet he was preparing for dinner.
He believes it may have been transported into his garden on pea straw he bought about a month ago from a Boundary Rd property, in a rural area between Wanaka and Mt Barker.
Dr Vink said he wanted the spider because he had not previously been able to get a live specimen from urban Wanaka.
"I've collected redbacks from around the base of Mt Roy and they have been reported around Central Otago since the 1980s," he said.
"I had heard of one reported in Wanaka, and this one will be good to have."
He recently published an article in the Biological Invasions journal about the distribution of the Australian redback spider throughout New Zealand.
Dr Vink and AgResearch colleague Dr Craig Phillips are developing new approaches and tools to ensure harmful organisms are kept out of New Zealand.
As part of this work, they record sightings of redback spiders in New Zealand, then use biological and climatic information to reveal where redbacks could become established.
Central Otago's warm, dry conditions would suit the spider but it was likely to spread further than expected, because they had the potential to survive in urban places with moderately high rainfall, such as New Plymouth, Dr Vink said.
Redback and other poisonous spiders are regularly intercepted by border controls.
Dr Vink said redbacks were not aggressive and no-one had died from a bite for many years.
They prey on insects.
Alexandra Doc biodiversity ranger Craig Wilson said redbacks also lived in the Cromwell chafer beetle reserve near Bannockburn and around Alexandra, "but people don't see them very often, so they can give people a bit of a surprise".
Theories about how the redback arrived in Central Otago included their being brought in with construction supplies for the Clyde dam or on grapevine root stock, Mr Wilson said.
Bites could cause excruciating pain and there was about a one-in-three chance of developing major symptoms from a bite, depending on the amount of venom the spider was able to release into its victim, Dr Vink said.
Bites can cause reddish lumps, which can be treated with rest and an ice pack.
Antivenoms are available at hospitals. However, bites are so rare that Wanaka's medical centres do not hold antivenom, because it would go off.
