The annual report of the Ombudsman, Professor John McMillan, which was tabled in Federal Parliament last week, said the number of approaches and complaints totalled almost 40,000.

Of the 19,412 complaints found to be within the Ombudsman's jurisdiction, 79 percent were about six agencies.

A total of 7226 people complained about Centrelink's handling of claims for Newstart Allowance, disability support pension, family tax benefit and the age pension.

The Child Support Agency was the second most-complained about body.

It was the subject of 24,761 complaints, an increase of 12percent on 2007.

Complaints against Australia Post rose by 6.5 percent to 2219, while there was a 5percent decrease in those against the Department of Immigration and Citizenship.

Most of the 1459 immigration-related complaints regarded detention centres, delays in refunding security bonds, security clearances and concerns about cancellation of visas on character grounds.

A total of 1422 people complained about the Australian Taxation Office, while there were 571 complaints about the Department of Education, Employment and Workplace Relations.

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