The World Anti-Doping Agency (WADA) stripped the $25 million (£17.6 million) Laboratorio Brasileiro de Controle de Dopagem (LBCD) of its accreditation in 2013 because it failed to meet required standards, re-instating it last year.
But unless Brazil changes its doping laws to conform to global regulations by the March 18 deadline, the anti-doping tests for the Games could be moved elsewhere at considerable cost.
"The lab will be fully accredited with the correct legal framework, which will mean full operations for the Games," Nuzman told Reuters at a news conference in London.
"All the wording has been approved by the IOC, WADA, the federal government and the organising committee. We prefer to do it by March 15. We expect to have this final document by then."
WADA rules say doping cases must be heard by an independent specialised tribunal as opposed to a general sports court, as is the current process in Brazil.
A Rio spokesman said Brazil's President Dilma Rousseff would sign a decree to change the law with immediate effect.
Doping is one of many issues dogging the build-up to South America's first Olympics, with Russia currently barred from international competition and Kenya struggling to implement new anti-doping measures by an April deadline or be ruled non-compliant.
NOTHING TO HIDE
Preparations for the Rio Games have also been overshadowed by construction delays, an ailing Brazilian economy and the Zika virus. In a further blow this week, French prosecutors investigating corruption in athletics raised questions about the bidding process for the Rio and Tokyo Olympics.
Nuzman said Rio had nothing to hide.
"We did it a clean way," he said. "The winning margin was large and we had strong support from a great number of (IOC)members."
With around five months to go until the opening ceremony, Nuzman said the only venue still lagging behind was the velodrome, which is still without its wooden track. The test event has been pushed back two weeks to April 30.
"This is the only test event delayed out of 44 test events," he said, adding that the golf course would be tested next week.
Nuzman also denied reports that athletes would be charged for mosquito screens for their rooms, saying the organising committee had agreed to pay for air conditioning to minimise the risk of being infected with the Zika virus.
The mosquito-borne virus has been linked to brain damage in thousands of babies in Brazil and both international health and sports officials are urging vigilance by those travelling to the Aug. 5-21 Rio Games.
Asked whether he was concerned that only 47 percent of tickets had been sold for the Games, Nuzman said Brazilians "loved to leave it late", pointing to the soccer World Cup.
But Nuzman said there were no plans to give away free tickets to children from low-income families.
"This is always a difficult situation," he said. "The organising committee decided not to have invitational tickets."
(Reporting by Martyn Herman; Editing by Gareth Jones)
