A dog alerted a council worker to a man who was lying seriously injured after riding his bike into a sinkhole in Waihi.

The man who fell into the sinkhole was Dave Keys, who relayed the story to his brother Ian last night.

"There was a bare open hole ... I guess in the dim light he didn't even see it. He just rode into. He said he had no idea what had happened ... next thing he was flying over his handle bars and he heard his neck break when he hit the ground."

Mr Keys could not move, and knew when the lights were turned off at the nearby netball courts for the night that he would be there for hours.

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"He knew that he was in trouble when he couldn't get off the bike. He couldn't move, he couldn't do anything," said Ian.

"Then when the lights went out he knew he was there for the night."

Mr Keys underwent a three hour operation on his neck today and will remain in Middlemore Hospital.

Mr Keys' niece Amber Keys is "heartbroken" by the freak accident.

"Praying for a miracle," she wrote on Facebook yesterday.

"My uncle has had an accident and it doesn't look good. Praying my heart out for Davey boy."

Today she posted an update, which said: "Thank you all for your lovely messages my girlies. He fell off his bike and broke his neck, paralyzed from neck down, very sad day but happy he is alive."

Dog stayed by injured man's side for hours

Hauraki District Council said Mr Keys was "conscious and talking" when found by their worker at about 4.15am.

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The dog belonged to Mr Keys, APNZ understands.

It stayed by his side over the seven hour ordeal.

It's understood that Mr Keys crashed into the hole about 9pm.

The park is a popular "shortcut" for Waihi locals heading in or out of the town centre.

Police returned the dog to Mr Keys' property where it is being looked after by family members, the council spokeswoman said.

The bin collector phoned 111 at 4.21am.

"It gave him a bit of a shock at the time but first aid training gave him the skills to cope with the situation," a spokeswoman said.

"He's OK, back at work and has been supported but has not required extra ordinary support."

The sinkhole was just 75cm deep, the council said.

Plastic barricades have been erected around it but it has not yet been filled in.

How the man was discovered

The council worker was alerted that something was wrong by the dog near the spot where Mr Keys was lying injured around 4.15am, Langley Cavers, chief executive of Hauraki District Council, said.

"Our street cleaner went into the area about 4.15am to empty the bins and noticed a dog, and he went over to investigate what the dog was doing, and saw a bike and found the man lying on the ground," Mr Cavers said.

"He could tell there was a potential for significant injury so he didn't move him, and called the emergency services."

Area prone to subsidence

The incident had occurred in a part of the reserve, close to a car parking area, which was prone to subsidence.

"The technical term is tomos, and we have had a few tomos in that area. Part of the area is an old refuse tip that's been sealed over," Mr Cavers said.

It was "not a regular occurrence, but it does occur", he said,

"In areas where it's been really bad we have landscaped them off, and they occur in the croquet courts next door and generally they're just filled," he said.

"So it's just a matter of when they occur if filling them in is appropriate, or landscaping them off, if it's an ongoing problem."

This particular sinkhole would be filled in, but council staff were still evaluating whether it may need to be landscaped in the long-term, Mr Cavers said.

"We're still investigating what's actually happened."

He described the incident as "a real tragedy".

Mr Keys was in a serious condition with neck injuries when paramedics arrived at the scene, St John Ambulance northern communications shift manager Norm Ngatai said.

The Westpac Rescue Helicopter was called around 5.30am, but could not reach the area because of fog, and was forced to land in Paeroa instead, Mr Ngatai said.

Mr Keys was taken there by ambulance before being flown to Auckland City Hospital.

A spokesman for the rescue chopper said Mr Keys was treated for hypothermia and suspected spinal injuries at the scene before he was taken to hospital.

Quick facts about sink holes:

• Sinkholes, known in New Zealand as tomos, can appear when ancient lava tubes were uncovered, when cave roofs collapsed, when water eroded buried sediment deposits, and possibly where old rubbish dumps were sealed over.

• Sinkholes commonly occur in karst regions, where slightly acidic groundwater dissolved the landscape's underlying bedrock, New Scientist magazine said.

• Ruptured sewer pipes can trigger sinkholes in urban areas.

• Last year, a Florida sinkhole swallowed local man Jeff Bush, who was sleeping in his bedroom. Mr Bush died.

• Sinkholes could not usually be predicted but in Florida, authorities used ground-penetrating radar and electrical imaging to guess where sinkholes were likely to form.

• Famous sinkholes include the Great Blue Hole, a submarine sinkhole off coastal Belize. It is a popular tourist attraction and diving spot.

- additional reporting Kurt Bayer of APNZ and Anna Leask of the New Zealand Herald