Serious Fraud Office investigators have executed a search warrant on a multi-million dollar building project in central Auckland.

Auckland Now received a tip this morning that the Hobson St Substation redevelopment, being undertaken by Hawkins for Transpower and Vector, was closed this morning and workers sent home.

When contacted, Hawkins speedily engaged a public relations company and released a statement saying the SFO search warrant was a "complete surprise".

SFO chief executive Adam Feeley confirmed a warrant had been executed at the premises of Hawkins but stopped short of naming the company as the target of the investigation.

"We have commenced an investigation into allegations of fraud as a result of information provided to us.

"We cannot at this time provide any further details on the nature of the allegations or the parties that are the subject of the investigation."

Hawkins also refused to say what was being investigated saying it was "very early days" and they did not have "a clear picture of the scope of the investigation".

Vector and Transpower were similarly tight-lipped with Vector releasing a statement saying it was "aware that the Serious Fraud Office has commenced an investigation involving a contractor on the joint Vector/Transpower project on Hobson Street".

"It's our understanding that the SFO investigation does not involve Vector or Transpower," a spokeswoman said.

"Vector and Transpower remain focused on completing this project so that security of supply into the CBD and North Shore will be enhanced."

The Hobson St project is part of a $46 million upgrade to increase "diversity" of power supply to central and north Auckland.

- © Fairfax NZ News