The strike began on Sunday.
An 8.5 percent production loss was registered on Tuesday, the third day of the strike.
Oil workers are seeking an 18-percent rise in their salaries. They've already rejected a Petrobras offer of an 8.1-percent raise.
Petrobras says in a statement that it's working to return production to normal. The company also says that tax revenues will be directly affected by the fall in production.