The Whangarei District Council has fixed two sinkholes which opened up last summer in roads in a major new subdivision.
The sudden slumping, after pipes cracked underground, raised fears that all the pipework in the Marsden development might fail, costing the council millions to fix.
The council has sent soil samples for testing to see if highly acidic soil and water could be to blame.
But developer Oliver Scott said it seemed obvious the cause was a few green, or uncured, concrete pipes.
He said the pipes were rejected by his engineers but signed off by the council after assurances from the manufacturer.
Mr Scott said the two holes appeared in the area where the suspect pipes were laid, and no more had appeared. If acidic water was a problem, the whole subdivision would be affected - not just a small part, he said.
PH levels in the water and soil had been constant since before the development began.
The council, which now owns the Marsden infrastructure, said it could not determine liability for the repairs until the test results were back.