A French expert looking at the Goro Nickel mining project in New Caledonia says questions remain about the waste disposal from the plant which is to start production later this year.
However, the expert from the French ecology ministry, Jean-Pierre Leteurtroiser, says alternatives to the pipeline are not viable.
The pipeline is planned into the lagoon in the south of the main island which is now a Unesco world heritage site.
According to the Noumea daily newspaper, he considers it impossible to have more effluent being evaporated naturally because of the heavy rain in the area.
The paper says to evaporate 1,200 tonnes of effluent an hour with another plant would require massive additional power generation.