TOKYO, July 3 (Reuters) - Japan's NTT Data Group said on Wednesday its Romania unit was hit by an instance of unauthorised access in June.

The company is investigating how the access, which occurred on June 14, happened and whether information was leaked, a spokesperson said.

NTT Data said the access was detected on an old network that the Romanian unit was no longer using as its main network.

It had acquired all of the shares in a Romanian information technology service company in 2013 to make it a subsidiary.

Shares of NTT Data fell as much as 6.55% on Wednesday compared with a 1.26% gain in the benchmark Nikkei index. (Reporting by Kaori Kaneko; Editing by Christopher Cushing and Muralikumar Anantharaman)