Catalysis of the reaction: 5,6-dihydroxy-3-methyl-5,6-dihydroquinolin-2(1H)-one + NAD+ = 5,6-dihydroxy-3-methyl-2-oxo-1,2-dihydroquinoline + H+ + NADH.
5,6-dihydroxy-3-methyl-2-oxo-1,2,5,6-tetrahydroquinoline:NAD+ oxidoreductase activity