SYDNEY--National Australia Bank Ltd. (NAB.AU) said first-half profit jumped 23% as earnings from personal and wholesale banking improved, but warned of rough seas ahead for the nation's economy.

In the six months to Mar. 31, net profit at Australia's fourth-largest bank by market value was 2.52 billion Australian dollars (US$2.56 billion), up from A$2.05 billion a year earlier, National Australia Bank said in a stock-exchange filing on Thursday.

Cash profit, a measure closely watched by analysts that smoothes out one-time items, rose 3% to A$2.92 billion. The figure beat the A$2.89 billion forecast by six analysts polled by The Wall Street Journal ahead of the result.

Group Chief Executive Cameron Clyne said there were "challenges remaining" for Australia's economy, adding that business and consumer confidence remained fragile even after steep rate cuts by Australia's central bank.

"It's a little too early to tell" whether the cuts have had any impact on the economy, Mr. Clyne told journalists on Thursday. "Consumers look beyond rate cuts just for a spurt of confidence. They look to the future as well, and there's still a lot of uncertainty in the global economy."

The most valuable of Australia's lenders a decade ago, National Australia Bank has lagged its peers since the financial crisis as its struggling U.K. business has eroded profit. Like other lenders, it's also been battling a slowdown in the domestic economy.

Cash profit from business banking rose 8.3% as bad debt charges fell by A$120 million from the immediately preceding half. Personal banking income climbed 19% from a year earlier on better margins and improving volumes.

The central bank Tuesday cut interest rates to a record low 2.75%. It was the seventh time it has lowered its benchmark rate since November 2011. The surprise move, aimed at spurring consumer spending as a mining boom that's powered growth for a decade cools, puts the official rate lower than during the depths of the financial crisis.

Still, Australia's banks, among the sturdiest in the world, have proven resilient to the slowdown. Last week, National Australia Bank's rivals Westpac Banking Corp. (WBC.AU) and Australia & New Zealand Banking Group Ltd. (ANZ.AU) reported consensus-beating first-half results, helped by improving profit margins, higher trading income and cost-cutting.

Both paid out large returns to shareholders--including a special 10 cent dividend from Westpac--sparking a jump in their share prices. National Australia Bank said Thursday it would pay an interim dividend of 93 cents a share, compared with 90 cents a year earlier.

Last year, ballooning sour debts at the bank's Yorkshire and Clydesdale banks dragged annual net profit down 22%, the first fall since 2009. The poor result, at a time when Australia's other major banks were reporting record results, triggered a backlash against executive pay at the group's annual meeting in December. Mr. Clyne pledged three months later that he would strip A$800 million from the bank's annual costs by 2018.

Bad and doubtful debt charges at the bank added up to A$1.09 billion in the first half of 2013. That was similar to the A$1.13 billion reported a year earlier but down A$142 million from the immediately proceeding half, thanks to lower charges at the bank's U.K. and Business Banking divisions.

Mr. Clyne said the restructuring of the U.K. business was proceeding at a faster pace than expected, with 1,100 of the planned 1,400 job cuts already made. Its U.K. bad debt charge was £185 million in the half, down from £197 million a year earlier.

"The U.K. is having the weakest recovery since the 1930s," Mr. Clyne said. "Were pleased with the U.K. as a whole. If the economy doesn't have any further deterioration then we have prospects in the U.K."

The lender has tried to squeeze more profit from its corporate loan book, the biggest in the country, which has been hurt by weak credit demand and rising bad debt among smaller companies. Things may improve if business confidence picks up and companies start borrowing more.

A survey by East & Partners published this month found 47.1% of small-to-medium-sized businesses planned to borrow from banks by the end of June, up from 44.2% a year earlier.

National Australia Bank has also been pushing hard into Australia's A$1.3 trillion home-loan sector by offering the lowest interest rates to customers of all the biggest four lenders. On Tuesday, it became the first to pass on the central bank's 0.25% rate cut in full.

Write to Caroline Henshaw at caroline.henshaw@wsj.com

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