Defending champion Roger Federer is seeded No. 3 after Rafael Nadal. Women's defending singles champion Caroline Wozniacki, who beat Halep in last year's final at Melbourne Park, is No. 3 behind second-seeded and 2016 champion Angelique Kerber.
Serena Williams, who won the Australian Open in 2017 while pregnant with daughter Alexis Olympia, returns as the 16th-seeded player after missing last year's tournament.
Fifth-ranked Juan Martin del Potro will miss the Australian Open while he recovers from a fractured right kneecap.