December's deficit, announced on Friday by the statistics bureau, was $150 million, narrower than November's revised $610 million and below what was forecast in a Reuters poll.
Cutting the deficit was an unexpected 5.5 percent decline in December imports from a year earlier. But exports were a surprise in the other direction, falling by 9.78 percent from a year earlier compared with the poll's forecast of a 2.59 percent drop.
"The key question is whether the December number represents a change in trend for the trade deficit - is the worst now over? We doubt it," wrote Credit Suisse economist Robert Prior-Wandesforde in a research note.
"In particular, the weakness of imports seems very odd, given the apparent on-going strength of domestic demand, and is unlikely to last," he said.
Prior-Wandesforde said that while exports should recover over time, "Indonesia's consumption and investment growth is likely to outpace that of most of its trading partners."
Central to last year's high levels of imports has been robust economic growth. Many economists believe that in 2012 Indonesia grew about 6.5 percent. The statistics bureau changed the date it will release the full-year data to Feb. 5 from Feb. 6.
A factory survey also pointed to surprising weakness in the manufacturing sector in January, though most economists say Southeast Asia's largest economy remains on track for another year of robust growth.
The HSBC Market survey showed manufacturing activity contracted in January for the first time since May, even though new export orders rose.
The January purchasing managers' index was 49.7, compared with 50.7 in the previous reading and a record high of 51.5 in November. A reading below 50 signals contraction in manufacturing activity.
Similar surveys also highlighted patchiness in other Asian economies at the start of 2013, with China's factory sector managing only a shallow rebound.
Deficit Pressures Rupiah
Indonesia always produced an annual trade surplus - until 2012. The emergence of monthly deficits and a growing current account deficit last year added to worries about the rupiah , which was the worst-performing emerging Asian currency in 2012.
December marked the ninth consecutive month that exports fell year-on-year. The October deficit, $1.54 billion, was a record high.
For all of 2012, the trade deficit was $1.63 billion. Behind the gap were weak global markets and sharp increases in some imports, such as machinery and fuel.
Exports fell 6.61 percent in 2012 from the previous year, while imports rose 8.02 percent.
"While the December trade deficit shrank, we don't exactly read today's data as encouraging at all given that exports remain in the doldrums for now," said Gundy Cahyadi, economist at OCBC Bank in Singapore.
"This continued to signal that the trade account may remain under pressure in the near-term," he said.
Uncertainty over the trade and current account deficits suggests the authorities are increasingly reluctant to let the rupiah fall too fast or too much.
"The authorities are set to ensure better dollar liquidity onshore going forward. This should provide some support for the rupiah despite the still gloomy outlook on exports," Cahyadi added.
The statistics bureau also reported a pick-up in inflation in January due to higher prices for food, beverages and tobacco products. The annual headline inflation rate was 4.57 percent, compared with 4.3 percent in December and a poll forecast of 4.5 percent.
During the month, Jakarta had massive floods that hit distribution in the capital.
The relatively low inflation, coupled with a still gloomy global economic outlook, means Bank Indonesia is likely to keep its benchmark rate unchanged at a record low of 5.75 percent at a Feb. 12 meeting. (Reuters)