The Engineering, Printing & Manufacturing Union says engineers responsible for about half of Auckland's power lines network are poised to begin industrial action over a stalemate in their pay talks.
About 80 workers employed by Electrix will limit their work from midnight.
Electrix is contracted by the lines company Vector.
Union members will indefinitely ban any overtime and limit their callouts.
The EPMU says Electrix wants its members to agree to a 30-month pay freeze as well as a cut in superannuation.
The union says the pay freeze is effectively a pay cut as inflation is set to rise and GST is also going up from 1 October.
It says Electrix will also benefit from a cut in the company tax rate announced in the Budget.
The EPMU says the Employment Court on Monday dismissed an application by Electrix for an injunction.
The union says its members will still respond to jobs where public safety is a concern.
Vector says it has contingency plans in place. The company says it can use other contractors to make sure public safety is not compromised and the network is reliable.
It says Electrix also has plans to respond to any event that may impact the lines network.