Kia's 28,000-strong union was staging a work stoppage for four hours during the day, with the official saying it will cost 13.8 billion won (US$14.8 million) in lost production.
On July 27, the union voted against the deal for a 5.2 per cent increase in basic salary and a bonus equivalent to one-and-a-half months of salary.
Including Monday's production loss, Kia estimated partial strikes would force it to have lost production worth 377.1 billion won so far this year, the official said on the condition of anonymity.
The union had originally demanded an 8.9 per cent rise in basic salary and a bonus equivalent to two months' salary.
Officials say both sides would meet later in the day for wage negotiations.
