Diesel production has already been established and disruptions on the domestic market have been avoided, energy minister Dubravka Djedovic Handanovic said in a press release on Wednesday.
The 95 million euro ($103 million) overhaul began in February and was due to be completed on April 17.
In December, NIS said it intends to make capital investments of 59.7 billion dinars ($552.4 million/510 million euro) under its business plan for 2024.
Russia's Gazprom controls 56.15% of NIS, followed by the Serbian government with some 30%, while the remainder belongs to minority shareholders.
$ = 0.9227 euro