Banks in Indonesia enjoy the highest net interest margins of any key economy in Asia, and consistently place near the top of the profitability rankings of global emerging markets. But a new rule from the central bank may dampen these margins; Jakarta-listed lenders have taken a beating in the markets this week as a result.

At first glance, the rule sounds innocuous: by the end of the year, banks will be required to publish their prime lending rates. However, in Indonesia’s relatively opaque banking market, this minor boost to transparency is expected to unleash heated competition as borrowers suddenly find it easier to shop around.