The 52-year-old remained in Middlemore Hospital's intensive care unit last night, where a spokeswoman said he was still in a serious although stable condition.
Hauraki District Council chief executive Langley Cavers disclosed yesterday that the accident scene was only about 200m from a sinkhole above underground mine workings into which a house slumped in 2001. Almost 40 other houses were declared unsafe for human habitation between then and 2006.
He also confirmed there had been several previous incidents of ground slumpage over the years in Morgan Park, behind netball courts where Mr Keys was unable to make himself heard by players training there.
"I think there has been a series of holes," he said.
"They've all been within about a 100-metre radius of that area - every two or three years one will pop its head up."
He also said the ground under the netball courts was settling.
But that was because of natural underground tomos, and there had been no mining under the park.
Nobody had been injured in previous incidents, and he did not believe mining would have caused any loss of groundwater.
Near-surface groundwater systems were monitored, despite being geohydrologically separated from deep aquifers drained by mining.
Mr Cavers there was no urgency to investigate the sinkhole into which Mr Keys fell, as it had been cordoned off since then.
"Over the next week, we'll have a look to see what caused it, and whether it could have been prevented sort of thing."
Ian Keys said on Friday the hole should have been identified earlier as an unlit blind spot in what was otherwise a well-maintained park.
But Mr Cavers said nobody had reported the hole to any staff to whom he had spoken.
"We'll kick off with our staff [today] looking at what we need to do and what resources we need to have a quick look at it.
"We'll most likely dig down to try to identify what caused it and if possible block off underground any little void where water's running through."
