There is nothing wrong with governments turning to external consultants for special expertise or fresh ideas on a tricky project.

But the explosion in the use of external consultants, highlighted by the NSW auditor-general, will add to concerns that core areas of government are being outsourced and the state is losing the ability to think for itself.

Commonwealth Auditor-General Grant Hehir is worried about the independence of advice.

The auditor-general's report found that spending on external consultants in the five years to 2016-17 was $1.5 billion, and the annual spend in that year was 58 per cent higher than in 2012-13. The state spent $327 million on consultants in 2016-17.

External consultants do not have the public service's history of independent advice and can be motivated more by maximising fees than providing good advice. One particularly worrying issue is that consultants will provide the advice that their customers want to hear. Major consulting firms overseas have been embarrassed by fraudulent advice to governments.