The amount of money spent on external consultants by key NSW government agencies is forecast to soar this financial year, prompting opposition claims it is a waste of taxpayer funds.

But NSW Treasurer Dominic Perrottet has defended the expenditure, arguing the government is embarking on unprecedented and complicated reforms which often require expertise outside the public service.

Treasurer Dominic Perrottet says consultants are needed for their expertise. Credit:Geoff Jones

Figures tabled in the Legislative Council show the consultancy budget for the Department of Premier and Cabinet is set to jump to $10.8 million, up from $3.5 million in 2016-17.

The Department of Planning and Environment's budget for consultants in 2017-18 is $11 million, up from just $1.1 million in 2016-17.