The possibility of a computer glitch was being investigated, as were other possibilities including conflicts between subcontractors and their employers.
The Minister for Communications and Information Technology, Amy Adams, who oversees the UFB rollout, said the issue came to light only late yesterday, and was being investigated.
"In terms of the ultra-fast broadband build, Crown Fibre Holdings are investigating," she said. "There is no reason to suggest there is going to be a significant delay to the build."
Ms Adams said she did not know which of the four New Zealand companies had hired Transfield.
The agency was putting strategies in place to support those affected and see the issue quickly resolved, she said.
The $1.5 billion UFB project, due to be completed by 2019, is part of a public/private partnership to bring fibre-optic cabling to homes, businesses and schools.