Experts warn Japan's medical system is not ready for an invasion of potentially deadly redback spiders, with limited supplies of the anti-venom available to authorities.

About a dozen of the venomous Australian spiders were found in a small park in a residential area of Mitaka in Tokyo, terrifying local parents.

The redbacks were quickly captured and exterminated by authorities but government official Motosugu Tanaka said they would not be able to stop them from spreading.

"It's unthinkable that they only exist in Mitaka, so they'll probably be found in many other areas of Tokyo from now on," Mr Tanaka said.

Dr Koichi Goka from the National Institute for Environmental Studies said the Japanese medical system was not prepared for the spread of the spider.

"The medical system is behind. They have anti-venom in Osaka and Fukuoka but other provinces don't have supplies. So it's a serious issue and hasn't been addressed," he said.

Japanese experts warn it is possible that redbacks could mutate in Japan and return to Australia with a stronger, more toxic bite.

The dangers of the redbacks, which are potentially fatal to small children, are well known in Australia, but in Japan they are not.

A school education program has started, teaching children: "Even if you see small spiders with a pretty red back don't touch them."

The venomous spiders were first detected in Japan in 1995, in the city of Osaka.

Since then the spiders have quickly spread to most parts of country, and experts say the population now numbers more than 10,000.

What has surprised many is that the Australian redback has survived the harsh Japanese winters.

Dr Goka said the spiders had found refuge in heating systems or even under heated toilets.

"They've thrived because they don't have any natural enemies here and they have plenty of food," he said.

The redbacks have bitten about 100 Japanese people, but so far there have been no fatalities.