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By Andre Romani

SAO PAULO, Aug 13 (Reuters) - Brazilian digital lender Nubank's quarterly net profit surpassed $1 billion for the first time, beating analysts' estimates and causing ‌shares to surge after the bell on Thursday.

Nu Holdings posted net profit ‌of $1.06 billion for the April-June quarter, a 49% rise year-on-year on a foreign-exchange-neutral basis and above the $967.2 million ​Visible Alpha estimate.

Shares in the lender, which serves nearly 139 million clients across Brazil, Mexico and Colombia and is preparing to start operating in the U.S., jumped some 9.5% in extended trading to about $15.25 each.

The profit increase was driven by higher revenue and an improvement in ‌risk-adjusted net interest margin, Chief ⁠Financial Officer Rob Livingston, who assumed the role last month, told Reuters.

Nubank's revenue increased 39% to $5.88 billion, beating the $5.60 billion projected in ⁠the Visible Alpha forecast, while the risk-adjusted net interest margin rose to 12.4% from 9.9% a year earlier.

Livingston told an analyst call that the current level of risk-adjusted net interest margin (NIM) ​is seen ​as sustainable in the foreseeable future.

"We believe bull ​investors were working with ~11% risk-adjusted NIM, ‌meaning this is a solid beat even for investors who were positive into the print," JPMorgan analysts wrote.

Cost of credit, which weighed on shares last quarter after rising to $1.79 billion, declined to $1.69 billion, though it remained 60% higher than a year earlier.

Nubank benefited from Brazil's Desenrola debt-refinancing program, launched this year to help individuals renegotiate debt, according to ‌the CFO.

However, he said the improvement would have ​occurred even without the program, citing seasonal factors and ​noting that Desenrola accounted for only ​about 5% of the bank's total cost of credit.

The credit portfolio ‌stood at $39.4 billion, a 37% growth year-on-year ​and a 5% rise ​quarter-over-quarter.

Credit portfolio "did slow a little bit sequentially in terms of the growth rate relative to the first quarter," Livingston told Reuters, but noted the move came from ​an unusually strong recent expansion.

Early ‌delinquency rates came in at 4.8%, up 0.3 percentage point year-on-year, but ​down from the 5% in the first quarter.

(Reporting by Andre Romani; Editing by ​Kylie Madry, Shri Navaratnam and Lincoln Feast)