TOKYO, Sept 6 (Reuters) - A malfunction that shut down all of Toyota Motor's assembly plants in Japan for about a day last week occurred because some servers used to process parts orders became unavailable after maintenance procedures, the company said.

The system halt followed an error due to insufficient disk space on some of the servers and was not caused by a cyberattack, the world's largest automaker by sales said in a statement on Wednesday.

"The system was restored after the data was transferred to a server with a larger capacity," Toyota said.

The issue occurred following regular maintenance work on the servers, the company said, adding that it would review its maintenance procedures.

Two people with knowledge of the matter had told Reuters the malfunction occurred during an update of the automaker's parts ordering system.

Toyota restarted operations at its assembly plants in its home market on Wednesday last week, a day after the malfunction occurred. (Reporting by Daniel Leussink; Editing by Edmund Klamann)