Aug 19 (Reuters) - Australia's Origin Energy Ltd on Thursday posted a 68% drop in annual underlying profit, and booked impairment charges of A$2.3 billion ($1.66 billion) from a collapse in wholesale electricity prices.

The electricity and gas retailer said underlying profit was A$318 million for the year to June 30, compared with A$1.02 billion last year. Analysts had expected A$275.6 million, according to Refinitiv IBES data. ($1 = 1.3816 Australian dollars) (Reporting by Tejaswi Marthi and Vasudha Kaukuntla in Bengaluru; Editing by Devika Syamnath and Chris Reese)