Organised as a fundraiser by the Cromwell Rotary Club, the tour of Brewery Creek downstream tunnel 650 attracted more than 500 visitors over two days, Sunday and yesterday.
"It's been a busy time. We've had everyone from great-grandmothers to babies in arms through the tunnel," tour organiser and Cromwell Rotarian Bill Wilson said.
The tours had been held at Easter for about a decade, usually every second year to coincide with the Warbirds Over Wanaka International Airshow.
"It's not just out-of-towners that come through, either.
"There's a lot of locals who say 'I've never been in these tunnels before and always wanted to see them'."
During the construction of the Clyde dam, numerous landslips were discovered along the proposed shoreline of Lake Dunstan and major remedial works were carried out at nine sites before the lake was filled.
Those works included a total of 18.5km of tunnels and 78km of drainage drilling, one of the tour guides, Cromwell Rotarian Murray Kennedy, said.
The aim was to offset the effects of lake-filling on the landslips. The tunnels and drain holes were built to drain groundwater within and below the landslip base and limit the rise in groundwater during lake fill.
The tunnels were used for geological assessments of the landslips, to remove water, and to install equipment for long-term monitoring of the site.
At the weekend, visitors could walk the length of the tunnel, past the shallow channel of water in the centre of the floor.
Mr Wilson said the novelty of touring a tunnel did not seem to wear off.
"I reckon our numbers are up a bit this year and there's been a steady stream of visitors."
The club uses the proceeds from the event to fund community projects, he said.
