Garden staff have closed off the steps to the lawn of the New Zealand native collection to protect the small bees' nest of tunnels.
The bees are also frequenting the dry garden beds of the North American borders.
Otago Museum entomologist Anthony Harris said the bees were the South Island species Leioproctus fulvescens.
Covered in dense orange-yellow hair, the bees did not sting and were often seen in summer carrying pollen on their back legs to feed their larvae.
They were common in the South Island, especially east of the Southern Alps, he said.
They preferred fine-grained soil for their nests, and usually preferred clay banks, but the dry conditions had probably created an ideal nesting site for them in the steps.
They dug nest holes in the ground, creating a small pile of soil at the entrance to individual nest tunnels.
New Zealand has 28 native and 13 introduced species of bee.
