* Cash investments put in risky credit instruments rated 'BBB-': * Cash returns wiped out by fees on some AMP pension funds (Adds quotes from lawyer and AMP spokeswoman, details on fund fees, misrepresentation of cash investments, earlier hearings on ANZ) SYDNEY, Aug 16 (Reuters) - Australia’s No.1 retirement and wealth management company AMP Ltd paid hundreds of millions of dollars from customers’ retirement accounts to its subsidiaries without proper documentation, an inquiry heard on Thursday. The disclosures are the latest blow to the once-venerable firm which could face criminal charges over misconduct earlier uncovered by the inquiry.