Inflation fell to 4.31 percent in September from 4.58 percent in August. The rate was forecast to rise marginally to 4.6 percent.
On a monthly basis, consumer prices rose only 0.01 percent. At the same time, core inflation came in at 4.12 percent, slightly down from 4.16 percent in August.
In September, the central bank maintained its key rate at 5.75 percent for a seventh consecutive month, saying the current rate is consistent with the inflation forecast.
The Organization for Economic Co-operation and Development (OECD) said inflation is likely to accelerate to 4.7 percent in 2013 from 4.2 percent this year.
In a separate report, the statistical office said exports plunged 24.3 percent annually in August and imports dipped 8 percent. The trade surplus totaled around $250 million, compared to an expected deficit of $0.06 billion.