HONG KONG, Oct 18 (Reuters) - Hong Kong"s Exchange Fund, which backs the local dollar, posted a third-quarter investment loss of HK$13.2 billion ($1.70 billion), its first after five consecutive quarters of gains, the Hong Kong Monetary Authority said on Monday. That compared with an HK$81.2 billion investment gain in the same period a year earlier. The Exchange Fund recorded investment income of HK$126.5 billion in January-September period this year. Among the investment income, the loss from Hong Kong equities was at HK$26.3 billion in the third quarter after three straight quarters of gains, and amounted to a HK$13.8 billion loss for the first nine months of 2021. HKMA said Hong Kong"s financial system is robust and the banking system has sufficient capital and liquidity. It also said it would ensure the stability of the local currency through the linked exchange rate system. The HKMA is the key manager of the Exchange Fund, which is under the control of the financial secretary and invests in equities, bonds, foreign exchange and other securities and assets. ($1 = 7.7780 Hong Kong dollars)