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Principal Reece Goldsmith confirmed this week the course, built in two stages in 1992 and 1994, was "completely destroyed" on Monday night.
Mr Goldsmith said it was a "devastating" loss for the school and there had been "tears shed" by "members of our school community".
The damage was now being cleared, "before our thoughts turn towards perhaps how we can rebuild it into an even better resource".
However, the cost of this would be "reasonably substantial", so significant investigations would need to be done into how to raise the money needed, Mr Goldsmith said.
School head of outdoor education Craig Gilchrist said the high ropes course was an "integral and important" part of Dunstan's outdoor education curriculum.
"Many students have been challenged and excited about it over the years."
Pupils were now "gutted" they would miss out on the "high ropes challenge course" part of their outdoor education programme this year, Mr Gilchrist said.
The school was having to make some "quick changes" to its year 9, 11 and 12 courses, as the high ropes course was going to be a large part of those years' outdoor education programmes during terms 3 and 4.
"Details are just being finalised, but we intend to offer some different outdoor activities in the short term and change the context for assessments to ensure the students still have a quality learning experience."