Google says emails are being restored to Gmail accounts temporarily emptied out two days ago.

Google planned to start moving backup copies of the missing emails back to their mailboxes around 4 p.m. ET Tuesday, the company reported around 2 p.m. on its App Status Dashboard, which updates users on service disruptions.

"In the hours that follow, users will regain access to their data," it said. "Accounts with more mail will take more time."

Early Tuesday morning, the dashboard reported no problems with Gmail, suggesting the missing data had been restored. However, the company posted later in the morning that only some of the affected accounts had been restored.

About 0.02 per cent of Gmail users had their accounts completely emptied. Gmail doesn't release official user statistics, but media outlets estimate there are roughly 190 million Gmail users, so about 38,000 were affected.

A blog post Monday evening by Ben Treynor, Google's vice-president of engineering, apologized for the problem and said Gmail should be back soon for everyone.

"The good news is that email was never lost, and we've restored access for many of those affected," Treynor wrote.

However, he noted that email sent to affected users between 9 p.m. ET Sunday and 5 p.m. ET Monday "was likely not delivered" — and won't ever be.

The sender would have received notification that the email bounced, Treynor said. However, those emails will remain permanently missing even after older emails have been restored to the accounts.

The post explained that while Google keeps multiple copies of user data at multiple data centres, in this case, a bug in a storage software update affected several copies of the data, deleting them.

The company had to restore the data from tape backups, which was a time-consuming process, the company said.

Following the outage, a number of software blogs have been recommending that users back up their email using offline mail readers such as Microsoft Outlook or Mozilla Thunderbird or by setting up Hotmail to receive copies of Gmail messages.