Reports from New Caledonia say hundreds of workers of the SLN nickel company have gone on strike and blocked access to the plant.
Public radio reports that there are burning tyres at the site as part of a union-backed protest against the possible closure of some furnaces.
The strikers fear that about 60 jobs could be lost.
The stoppage is also directed at the parent company's decision to delay building a new coal-fired power plant for the smelter.
SLN has been running at a loss because of the sharp decline of the nickel price.
New Caledonia nickel ore export policy decried
Meanwhile, New Caledonia's anti-independence party, The Republicans, has accused the government in Noumea of committing economic sabotage by blocking nickel ore exports to Japan.
The party, which is one of the territory's branches of the French opposition, responded to the government's refusal to allow the SLN nickel company to export one million tonnes of low-grade ore to Japan.
It says stopping such low grade exports prevents SLN from recovering some of the huge losses suffered because of the low nickel price.
The party says invariably decisions of this kind put directly or indirectly 10,000 jobs at risk.
It also points out that last month, Congress determined that exports to established customers should be continued.
The export restrictions are contentious and led to industrial unrest in August as truck drivers fear for their jobs in a depressed nickel market.
The government position has been that supplying the market with more ore is bound to contribute to keeping prices low.
Last month however, the government approved a bid by one company, MKM, to sell 300,000 tonnes of ore to China over the next 18 months.