The Tasman Glacier is retreating rapidly and was last week further cropped when a huge chunk of ice, estimated to cover a hectare, broke off.
Discovery Tours guide Elayne Nilson-Reid estimated the chunk of ice was 200m wide and 500m long.
She believed it broke off on April 1 following heavy rain.
Mrs Nilson-Reid told the Timaru Herald Tasman Lake undercut the glacier.
"What you end up with is like big shelves of glacier."
The firm's vantage point would shift when the broken-up ice floated away after the next big flood.
As the glacier retreats, tourists will travel further up the valley via the old 4WD track to the former Ball Hut.
The Department of Conservation no longer maintains the track and Mrs Nilson-Reid said if it was washed away four tourism and guiding operators would be affected.
"If we cannot keep that road open it means in future people will have to fly in costing several hundred dollars or walk for quite some time."
Glaciologist Trevor Chinn said the development of lakes was a tipping point for glaciers. No amount of snow at the head of the glacier, the neve, would compensate the effect of the lake.
Alpine Recreation guide Gottlieb Braun-Elwert estimated in 15 years the lake could have expanded to cut off traditional Mt Cook access via the Tasman moraine near the former Ball Hut.