Sam Rex-Edwards, from Finance Innovation Lab, a charity which campaigns for reform of the financial system, said she wanted to see mainstream lenders make affordable lending more accessible to those with low credit scores.
She was calling for a "fair banking" law that would place obligations on banks and other lenders to offer more support to those who struggle to get credit at the moment.
"Too many people are having to pay a poverty premium through using high-cost credit like payday loans," she said.
"And too many small and medium-sized businesses can't flourish because they're also refused credit from High Street banks."
There was some government backed support for small businesses from the British Business Bank and the Welsh Development Bank which both offer micro-loans for business start-ups.
The British Business Bank said it had provided "5,165 loans worth over £53m to start ups in Wales since 2012".
A UK Finance spokesperson said: "Last year we saw an increase in the amount of lending to small and medium enterprises in Wales, as well as more loan applications being approved.
"Banks also support other organisations, like non-bank lenders and charities, which help businesses that find it challenging to access mainstream finance.
"We are also working with government, regulators and business groups to help even more businesses access the finance they need to grow."