The Commerce Commission is reviewing a South Canterbury power company after it breached quality standards by exceeding power outage limits for two out of three years. 

In the commission's latest open letter to stakeholders, dated November 9, it identifies Alpine Energy as breaching standards, along with Vector and Aurora Energy.

The company says it is not concerned by the review, and the breach was not unexpected.

The breach arose after Alpine Energy exceeded its outage time in the year ending March 2016 by about a minute - having already breached the limit in the year ending March 2014. 

Alpine commercial and regulatory manager Sara Carter said the company was not concerned by the investigation. 

The breach was "not unexpected" and was the result of the company being slightly over its SAIDI limit. 

SAIDI is the average outage duration for each customer served across the course of the year, which in 2016 was 155.29 minutes. 

It was due to the weather, Carter said. 

"We suspected [we] probably would [breach]." 

READ MORE:  Alpine Energy defends condition of network after Commerce Commission report

"We literally just tipped over." 

The limit set for the year ending March 2016 was 154.16 minutes. 

In the 2014 to 2015 year, Alpine was well within its SAIDI limit.

However in the 2013 to 2014 year, it was more than 100 minutes over. 

Alpine prepared a report last June, available on its website, explaining the reasons for exceeding the 2016 limit. 

The report said two snowstorms on June 18 and June 19 2015, as well as a burnt cross-arm on April 12 and high winds on October 4 had affected the company's results for that year. 

"The 2015 snow storm was a one in 50 year storm that resulted in approximately 4,500 consumers, mainly in the Tekapo, Fairlie and upper Geraldine basin, losing supply on Wednesday, 18 June. Supply was restored to most consumers by the evening of Thursday, 19 June," the report said. 

The June 2016 report included a list of steps Alpine intended to take to shorten the outage time in the future, including a digital radio telephone to replace the current analogue system, establishing equipment caches for remote locations, and the increased use of smart meters. 

The report acknowledge 12 per cent of all outages were caused by defective equipment. 

A significant SAIDI outage was caused by an "insulator/disc" failure on the Mount Cook 33 kilovolt line. 

The report said the  Tasman River crossing section was scheduled to have all insulation replaced with superior line post clamp-top insulators in the 2016 to 2017 financial year. 

"In recent years we have addressed all other lines known to have these type of insulators." 

The most significant cross-arm failure was on the Pleasant Point 33 kV line. 

Consumers experienced interruptions of 8.10 SAIDI minutes due to "wildlife", with an "unusually high number of goose strikes" in early 2016. 

A Commerce Commission spokeswoman said the commission could not comment as the investigation into the breach was ongoing, and Carter said she had not been given an end date for the investigation. 

The open letter said another company, Nelson Electricity,  was also under investigation after it breached its price path.