Barnes, on a marketing trip for the popular Ecoya candles, said the star joined the house band about midnight after she'd had a few drinks.
"She looked pretty extravagant but was relaxed and having a good time. She sounded amazing when she got up and sang. She is a lot more of a talented singer than I thought."
Barnes said her friends were in Tokyo with the Jenesys Programme, which sends young Kiwis to Japan to forge business links. She is now trying to get a ticket for one of Gaga's New Zealand shows on her Born This Way Ball world tour. The concerts kick off at Auckland's Vector Arena on Thursday.
Gaga cancelled a performance in Indonesia slotted for today after a hardline Islamic group threatened to disrupt the concert, accusing her of being a "devil worshipper" and claiming her sexy antics were corrupting kids.
Dr Anwar Ghani, president of the Federation of Islamic Associations of New Zealand, said he was unaware of any plans for protests at this week's Kiwi gigs.
Lady Gaga sings at Park Hyatt Tokyo New York Bar: