A Perth father has joined calls for a royal commission into banking practices after the Commonwealth Bank trapped his son in a loan nearly triple what he needed, complete with punishing conditions.

Brian Borshoff, of Floreat, said his 20-year-old son went into the CBA for a $5500 loan for his first car and walked out with a $15,000 loan at 17.8 per cent interest.

Commonwealth Bank has been expanding quickly in the housing investor market, where bank growth is capped at 10 per cent per year. Credit:Glenn Hunt

Cancellation, early repayment or paying off more than the agreed monthly sum on the four-year contract was forbidden - meaning the total to be paid back is over $25,000 over four years.

The CBA staffer also offered insurance on the $5500 car at an annual premium of $1450 through the bank's insurance division CommInsure -the young man, however, declined and found a deal elsewhere for $900.