SYDNEY, Australia — Wall Street has started a bidding war for Fairfax Media, an Australian company best known for the dowdy business of publishing newspapers. To understand why, look no further than Deanna McMath.

Ms. McMath, owner of a small business specializing in print and design, is trying to determine the value of the fixer-upper house she bought in 2009 in the Sydney suburb of Stanmore, and whether to sell it and cash in on the area’s wild property boom. Where she once would turn to the real estate pages of The Sydney Morning Herald, a Fairfax paper, she now scours two online real estate portals: Realestate.com.au, which is part of Rupert Murdoch’s media empire, under News Corporation, and Domain, which has quietly become Fairfax’s most lucrative business.

“They’re the only two sites I go to,” she said. “There’s nothing in the papers. You don’t pick up The Herald anymore on a Saturday to see what’s for sale.”

Australia’s two biggest cities — Sydney and Melbourne — are having an extended, how-long-can-it-last surge in property prices, and for global investors, Fairfax’s Domain offers a piece of the action.