The moves comes after the commission asked the same court a number of legal questions in 2016, to clarify how the Credit Contracts and Consumer Finance Act (CCCFA) applied to the platform's fees and to consumer loans administered by Harmoney.
In May, the High Court struck out two of the commission's questions. The court is due to consider arguments about the remaining three questions in October.
The commission says the peer-to-peer lender breached the CCCFA, which prohibits lenders charging unreasonable credit fees.
The CCCFA requires that all fees are in proportion to the actual work involved in administering a loan. But before December 2015, Harmoney charged its fee as a percentage, ranging from 2% to 6% of the loan size.
The commission wants a declaration from the High Court that the two companies broke the CCCFA and wants compensation for borrowers who paid the percentage-based fees.
It said it had filed the proceedings to make sure that if the fee was determined to be a credit fee, that affected borrowers could be compensated.