A health fund will repay $620,000 in total to tens of thousands of members after an investigation found it had changed the rules on "fixed" dental benefits in likely breach of the law.

An Australian Competition and Consumer Commission investigation found health insurer Australian Unity had changed the way the annual dental limit on couple and family extras policies worked, even though it had made assurances the benefits were fixed for the year.

Originally, members under one policy could choose how they wanted to use and split the annual dental limit, which was between $1600 and $2400 a year. For example, all of it could be used on braces for one child.

After the change, each member was only allowed to claim up to half of the annual limit.