A fourth former National Australia Bank employee has been banned from working in financial services for his part in an alleged fraud ring that abused the bank's "introducer" home loan referral program.

Mathew Alwan was permanently banned from engaging in credit activities or providing financial services following an investigation by the Australian Securities and Investments Commission (ASIC).

NAB has run into yet more controversy. Credit:John Veage

ASIC is now considering whether to refer a brief for criminal charges to the Commonwealth Director of Public Prosecutions.

NAB believes about 2300 home loans since 2013 may have been submitted with inaccurate customer information and/or documentation such as payslips through its introducer program, where people outside the bank collect a fee for referring a customer.