Transpower's $5.5 million investment to improve the reliability of Hawke's Bay's electricity supply is needed because extreme weather events are happening more often, says Unison Group chief executive Ken Sutherland.

"Hawke's Bay customers have been impacted by several significant weather events in the past few years, such as snow storms, severe winds, and lightning strikes," he said.

In May 15,000 customers were without power due to Cyclone Cook, with some waiting nearly a week for power to be restored due to wind-blown debris shorting lines.

In January winds up to 160km/h downed power lines and in August last year dozens of power poles were brought down between Napier and Taupo by snow, leaving many people without electricity for several days.

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Transpower general manager of grid performance Jim Tocher said records showed extreme weather events appeared to be increasing in frequency, with four "substantial" events between 2012 and 2016.

"The transmission line between Wairakei and Whirinaki traverses terrain which is highly susceptible to lightning strikes close to Taupo and snow and/or high winds closer to Whirinaki," he said.

"We have analysed the reports from the previous events and have identified a number of improvements that we can make to help prevent or minimise the impact of these types of events on the region."

""We have worked closely with our customers in the Hawke's Bay region during our investigations and on the possible upgrades available. We believe that we have identified a good solution for the region and look forward to helping minimise weather related impacts in the future. We aim to have these improvements in place by May 2018.

"We will continue to work with Unison, Eastland Networks, Pan Pac, Genesis Energy and Contact Energy on investigations into future upgrades and alternative backup supplies for the Hawke's Bay Region," he said.

Mr Sutherland said Unison was already improving the resilience of its Hawke's Bay distribution network. Over the past year the lines company upgraded substations, placed major power lines underground and developed a "more proactive" strategy to prevent wind-blown trees, branches and debris taking down the network.

"These upgrades will help to improve the reliability and resilience of power supply for customers when faced with uncontrollable weather events, which we're seeing more often," Mr Sutherland said.

Work by Transpower will begin in July at the Redclyffe substation in Taradale and work on the transmission line from Taupo to Whirinaki is scheduled between March and May next year.