The Department of Conservation (DoC) has been forced to urgently decontaminate a dozen tramping huts after around 40 people contracted suspected norovirus.
Over the past week the gastro bug has spread through huts on an 80km tramping route in the Nelson Lakes National Park.
Coldwater Hut is one of 12 which have needed urgent decontamination.
Alarms were raised when several trampers walking the Travers-Sabine circuit an hour south of Nelson became sick just before New Year's Eve.
DoC's John Wotherspoon says all those who were sick displayed signs of norovirus. He and seven others spent Thursday morning cleaning huts.
The threat of gastro didn't appear to deter too many people, the local water taxi operator describing it as one of the busiest days of the summer.
Nelson District Health Board says no one is currently being treated for norovirus, but air ambulances are on standby if more trampers become ill.
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