Seek chief information officer Tam Vu said Hostworks could do a better job at managing its infrastructure, which was previously run in-house.
"We wanted our technical people to focus on solution development and service delivery for the business rather than managing applications and servers.
"Hostworks will be able to do this more effectively than us because they have the specialist skills, mature processes and infrastructure services," he said in a statement.
Adelaide-based Hostworks claimed "some of the world's largest IT services firms" had competed for the tender. Financial details of the agreement were not disclosed.
The CIO added Hostworks would help his company improve its business continuity strategy.
"Hostworks offered us a cost-effective disaster recovery solution," he said. "It allows us to get there in one go rather than do it over a couple of years as originally planned."
"Hosting of online business is more than just putting up a Web site," Vu added. "It is about providing an infrastructure that allows a business to live on the Web, so it must be a 24x7 service coupled with a highly secure environment."
Seek claims its Australian and New Zealand Web sites attract more than 1.4 million unique visitors each month, with an average of more than 80,000 job vacancies listed at any one time.
Other large local Web sites hosted by Hostworks include American Express, Sony, Ticketek, AAMI and ninemsn.