SYDNEY, Oct 20 (Reuters) - Australian free-to-air television network Nine Entertainment Co Holdings Ltd said its biggest shareholder, U.S. private equity firm Apollo Global Management LLC, plans to sell most of its stake to regional broadcaster WIN Corp.

In a short statement on Tuesday, Nine said the New York-based distressed buyout specialist would sell a 13 percent stake to WIN at an undisclosed price.

Nine's latest annual report said Apollo had a 22 percent stake, which means the sale will cut its holding to 9 percent.

Apollo and another U.S. private equity firm, Oaktree Capital Group LLC, bought Nine in two deals in 2006 and 2008, before downsizing their exposure in a 2013 listing.

Nine shares have traded below their A$2.05 issue price since June this year, and were down 0.3 percent at A$1.61 in morning trading, in line with the broader market.

WIN Corp also has a 14.9 percent stake in Nine free-to-air rival Ten Network Holdings Ltd, according to Australian media.

(Reporting by Byron Kaye; Editing by Stephen Coates)