Businesses are taking class-action over a power outage to of all Northland last year - although the exact loss is still unknown.

In June, an inexperienced worker removed nuts from three legs of a pylon, which toppled, cutting electricity to almost the entire region.

An opt-out class action against Transpower and its lines maintenance contractor is progressing through the courts. 

Stout Street Chambers partner, Mike Colson, says losses will be calculated in due course. 

"It's giving Northland businesses an opportunity to be compensated - and second, an opportunity to incentivise those working on critical infrastructure to do a good job out of it."