Taiwan has imposed restrictions on foreign investment in corporate bonds in a move seen as an effort to curb speculation in the island's currency.

The Financial Supervisory Commission (FSC) will cap overseas investors' holdings of company debt and bank debentures at 30 percent of their Taiwanese securities from Wednesday, according to a document sent to lenders and obtained by Bloomberg News on Tuesday. Previously, there was no limit. Such buyers' holdings of government bonds, money-market instruments, and derivatives are already restricted to 30 percent.

Taiwan's dollar has strengthened 1.7 percent against the greenback in 2015, in Asia's best performance, on speculation the central bank will refrain from joining a global wave of monetary easing. It has also rallied against Japan's yen and South Korea's won, harming the island's exporters who compete with their counterparts from those countries. Taiwan attracted $70 billion of net inflows in the first three months of the year, FSC data show.