The cost of paying a mortgage for most Australians has risen at its fastest rate in more than seven years despite the Reserve Bank keeping rates on hold for the last 16 months, according to figures released amid fresh claims the "big four" banks are exploiting loyal customers.

Cost of living data, released by the Australian Bureau of Statistics on Wednesday, shows interest charges on mortgages for employed Australians rose by 4.5 per cent since December last year - more than double the official rate of inflation and in defiance of the RBA cash rate remaining frozen at 1.5 per cent.

On Wednesday, a withering report from the Productivity Commission accused the "four pillars" of the financial services sector - the Commonwealth Bank, Westpac, National Australia Bank and ANZ - of gouging loyal customers by offering them higher interest rates than they do new customers.

The commission found loyal customers were "ripe for exploitation", with one in two people still banking with their first bank while accepting interest rates on home loans up to 0.4 per cent higher than new customers.