The Downer EDi Engineering and Transfield Services employees are contracted to provide line maintenance work for Telecom.
The strike could affect Telecom's phone, internet and eftpos services.
The industrial action comes as Telecom's network division Chorus moves to an owner-operator model following the signing of a 10-year contract with Visionstream in Auckland and Northland.
The Engineering Printing and Manufacturers Union (EPMU), which is representing the workers, said the workers were striking for redundancy protection.
EPMU campaigner Joe Gallagher said the issue was the "continuing desecration of the industry by Telecom".
"The guys have had enough," he told Radio New Zealand.
If the workers don't become owner-operators contracted to Visionstream, they will essentially be made redundant by Telecom, he said.
"In most cases they'll only get four weeks notice and then they'll be on the scrap heap, because most of these guys can't afford to go into this model."
The impact of the strikes could be felt in Auckland and Northland now, Mr Gallagher said.
"I'd imagine there was already faults building up...particularly in Auckland, since last week."
Today workers across the country would be picketing Telecom's move, he said.
Chorus's chief executive Mark Ratcliffe said not all line engineers would end up becoming owner-operators as, of the three companies Telecom has contracts with, only Visionstream works with the owner-operator system.
"Visionstream are taking over in Northland and Auckland, that's about 40 percent of the workforce."
The company's owner-operator policy was not the reason they won the Telecom contract, he said.
