Organisers have finally come clean about what caused an Olympic pool to suddenly turn green last week, and the answer is likely to prompt a chuckle from swimming pool owners.

After almost a week of murky explanations, officials explained that water at one of the pools at the Maria Lenk Aquatics Centre went from blue to green because maintenance crew had added the wrong chemicals to it. 

Maintenance crews added 80 litres of hydrogen peroxide to the water, neutralising the chlorine's ability to kill organic matter and prompting a chemical reaction, venues manager Gustavo Nascimento told the media. 

"The reason why [this happened], we haven't solved it. Our contractor's failure is our failure. It is something that should not have been done," Nascimento said. 

Officials will now transfer almost four million litres of water into the pool to ensure the synchronised swimming competition can start as scheduled on Monday.