Fitch said the group's holdings in the UK had weighed on earnings at NAB and left it facing risks some big rivals did not have to consider.
"NAB's UK operations leave it more susceptible to a downturn in Europe than its major Australian bank peers and this is reflected in the group's lower-than-peer profitability," said Fitch.
The remarks highlight the toll the downturn in the UK economy took on the performance of a group based in Australia, which has benefited from rapid economic growth in Asia.
In December, NAB's chairman Michael Chaney said the UK operation, which also includes Yorkshire Bank, had been a drag on the group in the year to September, when net profit fell 22% to A$4.08 billion.
Fitch added: "This exposure could leave NAB's asset quality more exposed to deterioration than domestic peers, particularly if there were a downturn in the UK, and negative rating action could result."
It noted Australia and New Zealand Group (ANZ) also faces risks that are less evident at "more domestically focused" Commonwealth Bank of Australia and Westpac.
Fitch affirmed the Long-Term Issuer Default Ratings for the four banks, including NAB, as AA-, with stable outlooks. It said the relevant ratings reviews did not encompass the group's overseas subsidiaries.
Last May Fitch affirmed Clydesdale Bank's long-term issuer default rating at A and said the outlook was stable.
The rating followed a strategic review of the UK operation by NAB, which decided to retrench to focus on retail and SME lending in Scotland and the north of England.
The bank has signalled this will take up much of 2013 but will leave it with a better performing and profitable UK business.
Earlier this month Clydesdale chief executive David Thorburn said the company has made good progress with a related restructuring.
Asked about Fitch's comments, Barry Gardner, media relations director at Clydesdale, said: "We're pleased NAB's strong credit rating has been affirmed with a stable outlook."