The Venture Southland promotion includes breakfast delivered by Invercargill Mayor Tim Shadbolt, who said he would not be making a bid in the auction.
"It is not for me."
However, privacy could be an issue, he said.
"It has no curtains."
Venture Southland tourism marketing manager Lisa Buckingham said the idea of the "tower of love" promotion was inspired by a Valentine's Day promotion offering couples a night in the Sky Tower in Auckland.
"We felt like Southlanders also deserved a romantic opportunity, and this will be the first time anyone has slept overnight in the water tower.
"The experience will be priceless and probably never repeated."
The winning couple could choose the date of their overnight stay, subject to agreement from the Invercargill City Council.
By yesterday, the highest offer on TradeMe was more than $150, and bids had come from as far afield as the Coromandel Peninsula.
The auction closes next Friday, with all proceeds going to the Cancer Society.
