Fairfax Media, which once dominated classified advertising in employment, will challenge online rival Seek through a joint venture with a UK-based job ad aggregator.

Fairfax, publisher of BusinessDay, on Thursday will launch the Australian arm of Adzuna, which compiles and lists job ads from across the web. The local site has more than 85,000 job listings since going live last May, and Adzuna Australia boss Raife Watson expects the number to double by the end of this year. A strong focus will be placed on recruitment agencies to deliver job seekers.

''Our focus is to work with recruiters to deliver them quality candidates in an efficient way that supports their existing operating model and helps them to grow their business,'' Mr Watson said.

Market leader Seek, which famously attracted much of the job ads once published by Fairfax and now has a market capitalisation almost three times that of Fairfax, had more than 111,000 domestic job ads in October according to the latest ASX filings. Seek declined to provide current figures.

The move adds fresh momentum to the battle in the online classifieds market. Fairfax's real estate arm, Domain, is gaining market share against News Ltd-backed rival realestate.com.au. At the same time, more entrants are fighting it out with US major eBay across the general classifieds market.

Fairfax chief executive Greg Hywood described the Adzuna deal as the ''disrupted becoming the disrupter''.

''We still have good front-of-the-book print ads which we make money from; in terms of our online offering, there's not a lot to lose. So we're playing a different game on it."

Launched two years ago by former executives at classified site Gumtree, Adzuna has more than 4 million monthly visitors across 11 countries and is backed by venture capital firms.

This is Adzuna's first joint venture, in a employment classified market it estimates to be worth $320 million this year.

Under the deal, Adzuna will promote itself through Fairfax's large print and online readership, and Fairfax will seek to win eyeballs and revenue by offering substantially more job vacancies through Adzuna's technology.

Unlike Seek, Adzuna is paid per job placement, rather than when an ad is listed. The price will depend on the difficulty of filling the role.

The Australian Adzuna site will be Fairfax's primary online job listings vehicle, with Fairfax's existing MyCareer brand to be renamed.

Mr Hywood described the deal as more strategically important than financially onerous, with both parties declining to reveal revenue targets or the price of the deal. Mr Hywood said it was ''not remotely expensive''.

Seek said there had ''always been competitors in the marketplace and the aggregator model isn't new to the Australian market''.

''We've got the resources in place to build on our leadership position through better searching and matching, making Seek the fastest and easiest way to find the most relevant job,'' Seek said.

Steve Allen, managing director of media research firm Fusion Strategy, said Seek had a huge first-mover advantage, and Fairfax would need a ''major outage'' on a Monday - the peak job search day - to seriously challenge it.

For the 2013 financial year, Seek said it had 70 per cent share of Australia's online job ad market, outpacing MyCareer and News Ltd's CareerOne.

At its annual meeting in November, it tipped a rise in revenue and earnings before interest, taxation, depreciation and amortisation for its domestic business this financial year, but said it would update the market to take into account trading in January and February.

Seek's share price has rallied since late 2011, and analysts have a 12-month price target of $13.25, higher than Wednesday's close of $12.94 a share.

Unemployment is tipped to rise to 6 per cent or more this year, with the government tipping 6.25 per cent.