Winder said a legal challenge would be based on the "reasonableness" of the targeted rate, which Mayor Phil Goff has proposed to fund spending by Auckland Tourism, Events and Economic Development(Ateed) to attract visitors and support big events.
"I don't see how anybody could conclude charging a sector 100 per cent of the costs of an activity where they derive 10 per cent of the benefits is reasonable," said Winder, who is also a former chief executive of Local Government New Zealand.
Last week, Goff unveiled a new version of the levy to appease industry and political concerns.
Under the latest version, hotels will pay more than motels and lodges; backpackers, camping grounds and areas on the fringes of the city will be excluded; and two zones have been created with different rates.
The rate is now expected to collect $15 million to $18 million, rather than the original $27.8 million. The targeted rate would free up to $18 million to fund transport and housing infrastructure.
Winder said the latest version strengthened the case for a legal challenge.
A mayoral spokesman said the revised proposal deals with concerns raised by the industry in a fair and equitable manner.
"The Mayor has received legal and technical advice throughout the development of the proposal and is confident the Council is legally entitled to levy the targeted rate," the spokesman said.
Auckland Chamber of Commerce chief executive Michael Barnett said watering down the proposed bed tax to just selected hotels doesn't change it from being unfair and inequitable and deserving of challenge.
Barnett said Goff was targeting a small group of ratepayers because he thinks they can afford it.
If the mayor needs $18 million, Barnett said, he should deduct it from the Ateed budget and take up the wish of Ateed chief executive Brett O'Riley to see a wider range of those in the tourism sector fund its work.