The ratings agency said high levels of consumer debt and high housing prices have left the banks more vulnerable to downside risks to the Canadian economy than in the past.
"Moody's recognizes the strong domestic franchises and solid earnings capacity of these large Canadian banks, and they will continue to rank among the highest-rated banks globally following this review," said David Beattie, a senior credit officer at Moody's.
The agency put Bank of Montreal (TSX:BMO), Bank of Nova Scotia (TSX:BNS), Caisse Centrale Desjardins, CIBC (TSX:CM), National Bank of Canada (TSX:NA) and Toronto-Dominion Bank (TSX:TD) under review.
Royal Bank (TSX:RY), Canada's largest bank, was not included on the list on Friday.
A downgrade by a credit rating agency usually means investors will demand a higher interest rate when a bank goes to raise cash by issuing bonds or other debt.
Moody's cut Royal Bank's long-term deposit rating to Aa3 from Aa1 in June as part of a move to cut the credit ratings of 15 of the world's largest banks, including Bank of America, JPMorgan Chase, Citigroup and Goldman Sachs.
At the time, the agency noted that RBC has stronger buffers than many of its global peers in the form of earnings from other, generally more stable businesses.
Both Bank of Canada governor Mark Carney and Finance Minister Jim Flaherty have repeatedly warned Canadians about taking on too much debt.
Earlier this month, Statistics Canada released revised data showing that household market debt has risen to 163 per cent of disposable income, well above the 152 per cent previously reported using a less focused measure.
The housing market in Canada has also shown signs of cooling.
The Canadian Real Estate Association reported last week that despite a slight recovery from August, home sales in September fell 15.1 per cent from a year ago due to tighter mortgage lending rules and an uncertain economy.
Moody's noted that its central scenario for Canada's economy is for growth between two per cent and three per cent next year, but the downside risks have increased.
The agency noted that a weak U.S. economic recovery, the ongoing crisis in Europe and a slowdown in emerging markets all weigh on commodity prices.
"Should these risks materialize, they would have significant ramifications for the Canadian economy that would be transmitted into the banking system," Moody's said.
In addition, the agency said National Bank, Bank of Montreal, Bank of Nova Scotia and CIBC have sizable exposure to the volatile capital markets businesses.
Moody's said TD is also exposed to the U.S. market, while Caisse Centrale Desjardins' concentrated franchise structure reduces its flexibility.
While Moody's said it was reviewing the long-term ratings, the agency affirmed its short term Prime-1 ratings on the six banks.
In July, Standard & Poor's Ratings Services revised its outlook from stable to negative on seven Canadian banks over concerns about unsustainably high home prices and consumer debt levels.
The debt-rating firm revised its outlook downward on Royal Bank, TD Bank, Scotiabank , National Bank, Laurentian Bank (TSX:LB), Home Capital Group Inc. (TSX:HCG) and Central 1 Credit Union.
The credit rater, however, reaffirmed the credit ratings on all seven banks.
S&P also affirmed its ratings and maintained stable outlooks on five other Canadian banks including CIBC and Bank of Montreal.
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