A dispute resolution service says unfair actions by some finance companies are harming the industry's reputation.
Financial Services Complaints says complaints have more than doubled from the previous year and are mainly about personal loans for vehicles.
Chief executive Susan Taylor says unlawful actions are common complaints and affect the vulnerable.
"Typically the consumer is alleging they've had the car or goods unlawfully repossessed."
She says typical complainants are vulnerable members of society - low income earners, beneficiaries and people for whom english is a second language.
Ms Taylor says a small proportion of lenders are letting the industry down and the complaints highlight a lack of financial knowledge.
