It could cost $12 million to end Sydney lord mayor Clover Moore's 12-year reign over the city.

That is how much it will cost ratepayers to implement new laws making it compulsory for 80,000 businesses to vote in council elections.

Changes to legislation, allowing businesses to have double the vote of local residents, have been widely tipped  to threaten Cr Moore's grip on the mayoral chains, when thousands of Liberal-leaning business votes flood next year's election.

Last election, only 1700 businesses voted out of 100,000 registered voters, even though businesses contribute about 80 per cent of the council's rates revenue.

City of Sydney estimates the cost of maintaining the non-residential register and rolls would be $7.82 million for 2015/16 and $4.21 million for 2016/17, according to a council report about the implementation of the business roll.

These costs were independently reviewed by PricewaterhouseCoopers.

Cr Moore said the legislation undermined the democratic process and was "completely unworkable".

"The cost is a scandal," Cr Moore said.  "To think ratepayers' money will be spent on a political gerrymander … it's undermining the democratic process, there is nothing  good about it [the legislation] at all."

Legislation forcing businesses to vote was introduced by the Shooters and Fishers Party and backed by the Baird government last year and requires there to be a continual, "accurate" register of non-residential voters.

The laws were introduced after a state parliamentary inquiry which recommended the council adopt a similar model to Melbourne, where it is also mandatory for businesses and landlords to vote.

Under the changes, the  City of Sydney Act 1988 requires the chief executive to "maintain and regularly revise the register to ensure that it is accurate".

Cr Moore said the strict requirement of an "accurate" register at all times was impossible to fulfil, leaving future elections open to legal challenge.

"It makes the future of the city very uncertain because this legislation could be challenged by an candidate that doesn't win an election. That's because it's impossible to keep the register accurate at all times," she said.

Chief executive officer Monique Barone wrote to the Office of Local Government, stating it was an impossible task to keep accurate track of all non-residential rate payers.

"Land ownership, leasing and occupancy situation across the city will be constantly changing meaning that it is impossible for the register to be entirely accurate at any given point in time," she wrote.

In his legal advice to the council, Bret Walker, SC, said the requirement for a "continual, accurate register was "highly impracticable" and he believed the legislation should be amended.

"It is quite apparent that any approach to preparation of the non-residential rolls will mean that they are out of date the day after they are prepared or updated," Mr Walker said in his legal advice to council.

"The breadth and depth of the exercise required (by the legislation) requiring an Orwellian conception of record keeping by government in respect of its citizens, borders on the impossible. At best it is highly impracticable," Mr Walker said.

A council report outlining the costs and recommendations will be debated on Monday night. Local government elections are to be held next year.