Queenstown resident Liam Jones said he spent about four hours inside the ageing Frankton hospital, where there is no air conditioning, last Thursday and was floored by the inside temperature.
During his stay he consumed 1.5 litres of water, he said.
''Even at the reception area it was roasting,'' Mr Jones said.
''Everybody was looking lethargic and sweaty and hot.
''I was in board shorts and a T-shirt and I was still sweating, not moving.
''People are in pain or feeling sick and you want to be at a nice ambient temperature - they're sat in there absolutely roasting, so it's making things even worse.
''These guys, they're just getting overwhelmed and overheated.
''You've got to take your hat off to them [hospital staff]. Working in those conditions they still kept everybody smiling, everybody was comfortable, the treatment was personal - you couldn't fault them at all.''
Local weather forecaster David Crow said Monday was the hottest day in the Wakatipu since records began in 1878, the mercury reaching 35.2degC.
Paula Jones, of Queenstown, told the ODT she spent about 45 minutes at Lakes District Hospital late Monday afternoon with her daughter.
Ms Jones said it felt cooler outside the hospital than inside.
''The doors and windows were all open, fans were on and we were sitting in the A&E part. There were six of us in total, and we were all just sitting there sweating our butts off.
''It was just horrible.
''Staff were great - they were doing the best they can and they were getting everybody through as fast as they can ... but I just felt really sorry for them,'' Ms Jones said.
