A firm led by Indian billionaire Gautam Adani will start in February to collect the data and biometrics of up to 1 million poor residents as part of the redevelopment of a slum in Mumbai that is considered one of Asia's largest.

The survey will be crucial in deciding who among the residents of the Dharavi slum will be eligible to receive a free home in the area that is being redeveloped. However, authorities have struggled for decades to fix up the area, which covers 640 acres (260 hectares), with Adani Group finally winning the bid to redevelop the area together with the state of Maharashtra, though legal disputes over the awarding of the contract are outstanding.

Only residents that have lived in Dharavi before the year 2000 will be eligible for the free housing. The last survey of the area was conducted 15 years ago and some estimates show about 700,000 ineligible inhabitants could be relocated outside Dharavi, which has sparked worries of lost livelihoods or high rent payments for those people.

In the door-to-door survey exercise the Adani-led firm will use questionnaires to gather details of Dharavi occupants, whether they use the premises for residential or commercial purposes, proofs of ownership and biometric data, said S.V.R. Srinivas, head of Dharavi Redevelopment Authority which is overseeing the project.

"Teams will go to each and every house ... biometric data will be collected," Srinivas said in an interview. "Objective is that all eligible people should get houses and no ineligible people should get undue advantage."

Indian opposition parties have protested the redevelopment saying the state government, headed by Prime Minister Narendra Modi's party, and other allies unfairly favoured Adani when the company won the $614 million redevelopment bid. The state and Adani deny any wrongdoing.

Adani has hired global teams for the project and Srinivas said he expects the redevelopment to start within a year.

The survey will be done in two parts with a pilot phase first executed within three to four weeks with a few hundred residents. The full exercise will take nine months.

The final eligibility of the residents who will get free homes or relocated will be decided by the Dharavi Redevelopment Authority, and more staff will be hired soon to oversee the survey and the project, Srinivas added.

(Reporting by Dhwani Pandya; Editing by Aditya Kalra and Christian Schmollinger)