Google says data has been wiped from disks at one of its data centres in Belgium - after it was struck by lightning four times.

The strikes resulted in permanent data loss in a small fraction of Google Compute Engine (GCE) storage systems.

Some affected disks later became recoverable. But some hardware which is more susceptible to power failure suffered irreversible data loss.

Data centres often need extra protection from lightning, say experts.

In an online statement, Google said that just 0.000001% of disk space was permanently affected.

"Although automatic auxiliary systems restored power quickly, and the storage systems are designed with battery backup, some recently written data was located on storage systems which were more susceptible to power failure from extended or repeated battery drain," it said.

The GCE service allows Google's clients to store data and run virtual computers in the cloud. It's not known which clients were affected, or what type of data was lost.

The company said it would continue to upgrade hardware to improve data retention and improve response procedures for system engineers during future incidents.

While four successive strikes might sound highly unlikely, experts say that lightning does not need to strike a building in exactly the same spot more than once to cause additional damage.

Justin Gale, project manager for the lightning protection service Orion, said lightning could strike power or telecommunications cables connected to a building at a distance and still cause disruptions.

"The cabling alone can be struck anything up to a kilometre away, bring [the shock] back to the data centre and fuse everything that's in it," he said.

And James Wilman, engineering sales director for the data centre consultants Future-Tech, added that while data centres were designed to withstand lightning strikes via a network of conductive lightning rods, it was not impossible for strikes to get through.

"Everything in the data centre is connected one way or another," he said. "If you get four large strikes it wouldn't surprise me that it has affected the facility."