Jan 15 (Reuters) - Shares of Australia's Tyro Payments Ltd plunged more than 12% on Friday after a short seller said the company had under-reported the extent of outages across its payments terminals over the past week.

Tyro on Wednesday said 30% of its 32,000 customers - the majority of which use a single terminal - were facing outages caused by a software issue, and that it was collecting 2,000 terminals a day to be repaired and returned.

Short seller Viceroy Research on Friday said it estimated around 50% of Tyro's terminals are offline based on its "extensive" checks with an undisclosed number of Tyro customers.

Shares of Tyro, the largest provider of payment terminals outside the so-called 'Big Four' banks in Australia, have lost nearly a third in value since the company first disclosed the outages last Thursday.

Tyro did not immediately respond to a Reuters request for comment on the report and trading in its shares were temporarily paused on Friday after the sharp drop pending an announcement.

The shares were last down 11.8% prior to the halt at A$2.32.

"Many stores we have called and checked have already switched providers," Viceroy said in its report, adding the fallout on Tyro's reputation and finances will likely be "severe and long-lasting."

In Tyro's announcement on Wednesday, it said outages faced by the majority of customers would be fixed by the end of the week while the remainder will be operational next week. (Reporting by Nikhil Kurian Nainan in Bengaluru; Editing by Christopher Cushing)