A pregnant Williams sealed her 23rd Grand Slam singles title and seventh Melbourne Park crown a year ago, but last week announced she wouldn't defend her title, saying she was "super close" but not quite ready to compete after giving birth in September.
With the 36-year-old having to wait to equal Margaret Court's all-time record of 24 Grand Slam singles titles, the tournament is there for the taking by a host of players, led by world number one Halep, second-ranked Wozniacki and fourth-ranked Elina Svitolina.
Last year there were two first-time major-winners while Serena was on maternity leave -- Jelena Ostapenko at the French Open and Sloane Stephens at the US Open.
But their chances in Melbourne look slim: Stephens hasn't won a match on tour since her Flushing Meadows triumph, and Ostapenko was far from her attacking best in her first-round defeats in Shenzhen and Sydney in the past week.
Wimbledon champion and world number three Garbine Muguruza retired with severe cramps in Brisbane last week, in what could be a further boost to Halep's hopes of a maiden Grand Slam.
The 26-year-old Romanian is full of confidence after lifting the title at Shenzhen in dominant fashion, but will need to overcome her sorry record at Melbourne Park where she has lost in the first round in both of the past two years when seeded in the top four.
Wozniacki is riding high after her resurgence in 2017 when she reached eight finals, with victories in Tokyo in September and at the season-ending Tour Championship.
The Danish former world number one lost in the Auckland Classic final on Sunday to Germany's Julia Goerges, but said she felt great heading to Melbourne as she too looks to make her Slam breakthrough.
"I've got a lot of matches under my belt this week, it was the preparation I hoped for," said Wozniacki, who is back up to second in the world and, if results go her way, can return to the top spot after a gap of six years.
In-form Svitolina last week won the Brisbane International after picking up five WTA Tour titles last year, more than any other woman.
The Ukrainian believes hard work in the off-season is paying dividends. "I've started to play more consistently," she said. "I'm stronger physically. I have a different look to my game."
World number six Karolina Pliskova lost to Svitolina in the Brisbane semi-final but also will be a contender for a maiden Slam behind one of the biggest serves on tour.
Britain's world number nine Johanna Konta could figure despite a slow start to the season. She reached the last eight in both of her Australian Open appearances to date, but slumped out of the Sydney International in the first round this week.
But the most dangerous floater is 2008 champion Maria Sharapova, who on Monday moved back into the world's top 50 for the first time since returning from a 15-month doping ban.
The controversial Russian, ranked 47, lost in the Shenzhen semi-final to Katerina Siniakova and admitted she had "a lot of things to improve on".
If it is not to be a new name on the trophy, it could well be the oldest player in the field -- evergreen 37-year-old Venus Williams.
Venus last won a Slam in 2008 but she enjoyed a renaissance last year, when she reached two Grand Slam finals and returned to the world top five.
Only sister Serena prevented Venus from winning an 11th Grand Slam singles title in last year's all-Williams Melbourne final. Victory would see her eclipse Ken Rosewall as the oldest player ever to win a major.