A cave in a national park near Nelson, will close for a year in an attempt to save the country's largest spider.

The rare spider known as the Nelson Cave Spider has a leg span of 13 centimetres and a 3 centimetre long body.

Since 2018 only one egg sac has been seen in the Crazy Paving Cave in Kahurangi National Park, prompting action to be taken.

Te Papa spider expert Phil Sirvid says it's a pretty spectacular animal in its own right.

He says losing the spiders would be a loss of an interesting and unique part of our fauna - as they only exist in that little corner on the South Island.