The second-seeded Mertens, who won her maiden singles title in Hobart last year as a qualifier, became the first player to win the tournament twice.
The match took more than seven hours to complete due to a delayed start and three suspensions of play for rain, the last coming after Buzarnescu had saved two match points with the 22-year-old world number 36 leading 5-2 in the third set.
"It rained like three of four times so I am really thankful everybody stayed actually," Mertens told reporters.
"It's been a wonderful week and to come back to Hobart is such a great feeling. My level was there during the whole week, so I'm really happy with the trophy."
The Romanian, a first-time WTA finalist, saved more championship points and held her serve when play resumed before the defending champion served out the match that lasted two hours and 29 minutes in total.
"I was really close," Buzarnescu said, upset with her high number of unforced errors. "I should have been more patient, and not complain that much."
(Reporting by Sudipto Ganguly in Mumbai; Editing by John O'Brien)
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