"I don't know much about him, but I know he's a fighter," Pacquiao told local media.
"I haven't watched his fights yet, but I'm going to get his last three fights and watch it.
"My message for him is to work hard, and make sure you're 100 percent conditioned on July 2 so we can give a good fight, we can entertain people."
The fight against Horn (16-0-1) will be the 38-year-old Pacquiao's first since his unanimous decision victory for the title over American Jessie Vargas in Las Vegas last November.
Pacquiao had previously agreed to an April fight against Briton Amir Khan in the United Arab Emirates but his promoter Bob Arum said last month it had been postponed after the $38 million (£29.72 million) offer for the bout failed to materialise.
The Horn-Pacquiao fight is eagerly anticipated in Australia and will be the biggest boxing event held in the country in decades.
Organisers expect a crowd of over 50,000 to cram into Lang Park, which regularly hosts domestic and international rugby and soccer matches.
Dean Lonergan, a director at promoter Duco Events, hoped the bout would revitalise boxing in Australia.
"It's been in the doldrums for some time," Lonergan said in comments published by state media ABC.
"I think you'll find this will be the biggest fight in Australian history and you'll see the underdog (Horn) come through, and there's nothing like Australians supporting their own guys when they're underdogs."
(Editing by Peter Rutherford)
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