A key Government watchdog is struggling to keep up with its oversight workload.

The annual report for the Office of the Ombudsman shows the agency is facing challenges with a significant workload.

In the last year the office received more than 12,000 complaints - the second highest it has ever received in a year.

The Ombudsman only managed to complete 50 per cent of urgent investigations within four months and just a quarter of priority investigations within six months.

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Almost half of its total complaint investigations weren't completed within a year.

All the results fell well short of the office's benchmark targets.

The report also showed the Earthquake Commission remained a hotbed of discontent when it came to its observation of the Official Information Act.

The Ombudsman's figures showed the EQC was the subject of 474 complaints to the Office of the Ombudsman last year.

Prior to the Christchurch earthquakes, the Ombudsman averaged 10 to 15 complaints about the EQC annually.

However, it appears the number of complaints is trending down - the 474 recorded last year was down from 649 in 2013-14 and fewer than the 838 recorded in 2012-13.