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Half of the $12,000 pontoon collapsed and was submerged, off the lake front at the northern reserve, although children were still sitting on and swimming off it.
Harbourmaster Marty Black said a diver had inspected the structure after a jogger reported the damage yesterday morning.
Wear and tear following weeks of poor weather was believed to be the cause.
The cost of repairs was estimated to be about $3000 and it was unlikely to be usable until contractors were back in business after the New Year.
Mr Black said while the pontoon did not pose any danger to swimmers, it would be removed today.