The big four banks' average customer satisfaction ratings have dipped to the lowest level since mid 2013, after peaking mid-last year, new Roy Morgan survey figures show.

As banks face heightened political pressure and Labor calls for a royal commission after a run of scandals, figures that are closely watched in the industry show the share of customers who are satisfied with their bank is dwindling, albeit from a high starting point.

The proportion of customers who told Roy Morgan they were satisfied with their lender slipped to 79.5 per cent in June, and has now fallen in five of the past six months, after reaching an all-time high of 81.5 per cent in May 2015.

The decline comes after a decade of steady improvements in banks' customer satisfaction ratings and, surprisingly, the dip has been driven by home loan customers, who are are being charged low interest rates.