Roy Morgan Research shows overall satisfaction with the big four banks dropped for a fifth consecutive month in June.
NAB was the only major lender to improve its satisfaction rating with customers in June, according to the industry's benchmark survey.
It is down almost two per cent since January, where it peaked at 77.8 per cent.
ANZ has had the largest drop in satisfaction ratings, falling more than five per cent to 73.2 per cent - its lowest level in five years, possibly due to out-of-cycle rate hikes in recent months.
Despite rate cuts in May and June, home loan customers have lower satisfaction levels than any other customer groups.
Head of personal banking at NAB Lisa Gray yesterday defended NAB's strategy to grow its mortgage market share by more than twice the industry average.
"The latest data released by the Reserve Bank for system growth on May has the NAB personal bank growing above system and faster than any other major bank," Ms Gray said.
"Our asset quality has improved over this time and new mortgages are return on equity accretive to the group.”
Ms Gray said NAB would, in the next few weeks, sign up its one millionth new customer since it launched the "Break Up" campaign in February last year.