Once again gravel has had to be used to shore up the damage, but the half-constructed house may have to be demolished to properly remedy the botch-up.

At a nearby Onehunga site, contractors had dug away underneath the deck of the brand new house next door.

The property owner was forced to get 15 truckloads of gravel in to shore up the dangerous work.

The neighbours got a shock when they came out one morning to find their section slipping away.

Work on this Onehunga site had to be stopped after contractors excavated underneath the neighbour's retaining wall.

Sub-standard earthworks on new housing sites in Auckland are threatening neighbouring properties and costing homeowners thousands.

Auckland Council says it is shutting down around two sites a week because of dangerous excavation work.

The problem is increasing as the city intensifies and tricky in-fill housing sites suddenly become worth building on, it says.

Auckland Council The neighbours got a shock when they came out one morning to find their section slipping away.

In one recent case in Onehunga, the neighbours were shocked to wake up one Saturday and find their section falling away.

The contractors on the adjacent building site had dug out underneath the concrete foundations of the retaining wall between the properties.

"They sent their dog into the back yard in the morning, and the dog disappeared into a big hole," Wolfgang Nethe, team leader building compliance, said.

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The council immediately issued a dangerous building notice and ordered the owner of the building site to shore up the wall. "We got them to tip 15 truckloads of gravel against that wall. On a Saturday."

He estimated the damage will cost at least $35,000 to fix, and building of the new house will be delayed until it's remedied.

He has seen sites where botched excavations have cost up to $100,000 to fix, he said.

While Nethe was working at the Onehunga property another neighbour arrived and told him about a different site down the road.

In that case contractors had dug out underneath the deck of the brand new house next door.

Again the project had to be stopped, with temporary bracing and backfill put in. Some of the half-built house may have to be demolished in order to get machinery in to replace the retaining wall, he said.

"It was a silly attempt to cut corners and cost which in the end is backfiring really badly.

"It's just stupid, it's irresponsible."

The problem was partly because the hot Auckland property market made it worth developing more difficult sites, the council inspectors said.

"We're basically now building on sites that were left over previously because they were regarded to be too expensive to build on because of the slope and the lay of the land," Nethe said.

"That in combination with contractors that are not risk averse, that sometimes don't have the knowledge and vision to see what they're causing.

"They might be very competent digger drivers but they haven't got the skills to assess geotechnical conditions on a site," he said.

The work needed to be overseen at all times by a competent project or site manager, the council said.

"What you often find is they do the excavation and at the eleventh hour do a final cut which goes right up to the boundary," manager compliance building control Barry Smedts said.

"I think it's just trying to make their building site easier to manage, in their eyes."

The concern over earthworks comes amid wider council worries about sub-standard building work in the city.

Auckland Council is currently failing around a third of building inspections, and has appointed a person full time to handle complaints against builders.

As the building boom gathers pace tradespeople with greatly varying levels of skill have flooded into the industry, and experienced project managers are busy overseeing multiple jobs at a time, it says.

The Home Owners and Buyers Association of New Zealand (HOBANZ) shared the council's concerns, chief executive Roger Levie said.

"There's so much work out there, that the usual subcontractors that some of the good builders or developers may be using are not always available when they need them," he said.

"So they're having to call in other people who aren't necessarily as experienced or skilled, and then they don't have the resources on site to do the supervision that's required to ensure that that work is up to standard."

There was a massive lack of capability in the site management area, Levie said.

"We know that the good contract companies are out trying to poach each other's good site managers and project managers."

One of the answers was better co-ordination between council and government officials and the industry, he said.

Within the last week HOBANZ had held a forum with the council, the Ministry of Business, Innovation and Employment (MBIE) and large building contractors.

In that instance it was focused around big remedial projects such as leaking buildings, but the issues were similar.

There was a need to work more closely with the council, he said.

"We have a very 'them and us' finger pointing position, so a lot of the blame gets put on the council. And yet often they're the ones who step in when things go wrong," Levie said.