Four tourists who broke into the city's storm-water network on Thursday night could have been killed by a dangerous build-up of hydrogen sulphide gas, and the Dunedin City Council has publicly chastised them for their ''extreme foolishness''.
Entry into storm-water drains is known as urban spelunking or draining, and is illegal in most countries.
Groups devoted to the practice have sprung up around the world since the 1980s and there have been many fatalities around the world where people have been overcome by toxic gas from sewers.
A police patrol car noticed three people standing around an opened drain outside Toitu Otago Settlers Museum about 10.15pm on Thursday night and it emerged a fourth tourist, a female in her 20s, was missing underground and was unable to be contacted for about 15 minutes.
Police were concerned she could have been injured or overcome by toxic fumes, and were about to launch a search and rescue party, when she returned to the surface unharmed.
Dunedin City Council acting water and waste services manager Gerard McCombie said she was lucky not to have been seriously injured or killed, and he was critical of the tourists' actions.
''It's potentially very dangerous - they could have encountered gases like hydrogen sulphide.
''And when it rains here, the storm-water drains can fill up very quickly and people could end up drowned.''
''If they got into trouble down there, it could be fatal - that's the issue with entering any confined place,'' Mr McCombie said.
When council staff went into the storm-water or sewerage system, they followed strict procedures to manage the risks, he said.
Staff were highly trained and equipped with gas detectors, personal protective equipment (suits, boots and helmets) and sometimes breathing apparatus, he said.
''For members of the public or untrained people to do this, it's quite silly and dangerous. We view it as an extremely foolish and dangerous thing to do.
''And to send rescue crews down there ... Emergency services staff have enough to do without rescuing people indulging in foolish behaviour.''
Mr McCombie believed even guided tours of the city's underground network would be too dangerous.
''Sewers and storm-water drains are not recreational areas. They are only for trained professionals.
''Tourist operations in other cities may carry special gear. No-one does that here.''
He was also concerned the tourists were effectively breaking and entering Dunedin City Council property, and he had expressed concerns to the police, Mr McCombie said.
However, Acting Senior Sergeant Nathan White said no charges would be laid.
''The reality is, it was a one-off event and it's not likely to happen again.''
The Australian Cave Clan website says members go draining because they are ''addicted'' to exploring, and like to see a cross section of what is underground, while others like to record sounds and take photos. All have a sense of adventure, the website says.
A small subset of explorers enter sewers, which are among the most dangerous locations to explore because of the risk of poisoning by build-ups of toxic gas such as methane and hydrogen sulphide.
The website says draining has a specialised set of guidelines, the foremost being ''when it rains, no-one drains''.