Mike Baird's $77 billion state budget is more than double what his predecessors had to spend just 15 years ago.

Annual expenditure by the NSW government has ballooned by $41 billion since 2002-03, a report on trends in the state's finances by the NSW Parliamentary Research Service shows.

Gambling taxes flowing to the NSW coffers have swelled by $1 billion in that period. However, the state has become a little less dependent on gaming revenue over time. Back in 2002-03, gambling and betting taxes made up more than 9 per cent of the revenue collected directly by the state government but that's only expected to be 8 per cent this financial year.

By far the biggest source of revenue for the NSW government is the GST, which is collected by the Commonwealth and allocated to the states. This year it will contribute almost $18 billion to the state coffers, up from $9.9 billion 15 years ago.