The figure, based on an analysis by the NSW fire fighters' union, is 2?? times the $185 average touted by the state government when it unveiled the reform earlier this month.

The analysis reveals homeowners in North Sydney, Mosman and the northern beaches face paying the higher average levy as they prepare to vote in the North Shore and Manly by-elections on April 8.

Treasurer Dominic Perrottet has introduced legislation to Parliament that shifts the bulk of the funding of the $950 million annual fire and emergency services budget from a tax on insurance contracts to a levy on all NSW land from July 1.

Homeowners will be able to calculate the exact size of the levy from May 1 after the fire and emergency services budget is set.

But announcing the reform, Mr Perrottet said the average bill for residential property owners would be $185.

The government says for fully-insured homeowners the fire services levy contribution should drop from an annual average $233, for a saving of $47 a year.

However, the Fire Brigade Employees Union analysis shows the highest residential payments will on average be $471.

It says homeowners in areas including Parramatta, Canterbury, Ku-ring-gai, Bankstown, Burwood, Canada Bay, Hornsby, Ryde and Strathfield face annual bills of $361.

For homeowners in Sydney's west, including Liverpool, Penrith, The Hills, Campbelltown, Fairfield, Hawkesbury, Blacktown, Blue Mountains and Camden, the bill is estimated at $224.

A spokesman for Mr Perrottet said the union figures "do not appear to accurately reflect the amount of FESL that property owners will pay" but did not release government estimates.

The union also argues that the burden of funding will shift towards residential land owners and away from business under the reforms.

A 2011 Insurance Council report said residential property owners contributed 45 per cent to the three-quarters of the fire and emergency services budget raised through insurance contracts under the existing system. Commercial property owners contributed 49 per cent.

Under the changes, the residential component is 58 per cent and for commercial land 26.6 per cent.

Mr Perrottet's spokesman said the government figures were based on "far more rigorous and comprehensive data" than the "estimates" in the Insurance Council report.

FBEU state secretary Leighton Drury said the levy was "a new billion-dollar tax on property owners that will cost many NSW households hundreds of dollars more".

Mr Drury said a government promise that land owners can calculate the exact amount of their levy from May 1 was too late as "the Berejiklian government is trying to ram this through the Parliament now".