NAB's Advantedge business received the highest number of consumer complaints among COSL members in 2010-11, according to the body's latest annual report.
In the annual operations report, the Credit Ombudsman Service listed the number of complaints against its members, though it cautioned that size does make a difference.
Mortgage aggregator Advantedge led the pack with 74 complaints over 2010-11. A total of 41 complaints were resolved, with 10 (or 25%) in favour of the complainant.
Despite a small market share, non-bank lender Liberty Financial came in second, receiving 44 complaints. 49 Liberty complaints were closed, eight in favour of the complainant.
Advantedge general manager of broker platforms Steve Weston said the result proved Advantedge's customer service record, given the size of the business.
"Advantedge has approximately 1.2 million borrowers within the wider business," Weston said.
"Given the nature of the business there will always be a small number of complaints, and to only have ten resolved in favour of the complainant given the scale of the business is a very good result.
"We always strive to provide the highest levels of customer service and the complaint numbers confirm that is the case," Weston said.
Many other bank lenders have complaints administered by the Financial Ombudsmans Service, making it difficult to make a comparison of complaints across the broader mortgage industry.
The Ombudsman has warned to take the numbers with a grain of salt.
"The number and nature of complaints we receive about our members are influenced by a number of factors, including the size of the member’s business (our larger members will generally receive more complaints than our smaller members), and the period of the member’s membership with COSL," the report said.
"Some members may not have been members of COSL for the full membership year [and] our complaint numbers are actual figures and have not been adjusted to take this into account."
Complaints can also snare companies only marginally related to the complaint in question. COSL said some complaints may have been recorded against a member even though they relate to the actions of another organisation, simply because the member was contracted to manage some aspect of the organisation's business or the member acquired the business.