HONG KONG, Feb 15 (Reuters) - Hong Kong's de-facto central bank bought HK$14.868 billion ($1.894 billion) from the market in New York trading hours to stop the local currency weakening and breaking its peg to the U.S. dollar, bringing the aggregate balance to below the HK$100 billion mark.

The Hong Kong dollar is pegged to a tight band of between 7.75 and 7.85 versus the U.S. dollar.

The aggregate balance, the key gauge of cash in the banking system, will decrease to HK$77.005 billion on Feb. 16, an HKMA spokeswoman said on Wednesday. ($1 = 7.8499 Hong Kong dollars) (Reporting by Donny Kwok; Editing by Chris Reese)