Law, who landed his maiden European Tour triumph with a dramatic victory in the Vic Open, opened with a two-under-par 70 at Lake Karrinyup.
That was matched later in the day by fellow rookie MacIntyre, leaving the pair sitting joint-16th at the end of the opening circuit.
Law carded four birdies and two bogeys while MacIntyre followed 11 straight pars by picking up three birdies in four holes before dropping a shot at the 17th.
On a day when Norway’s Kristoffer Reitan and Miguel Tabuena from the Phillipines set the pace with matching 65s, Liam Johnston was the only other Scot to break par with a 71.
The top 65 and ties after the second round will progress to the third round, after which a second cut will reduce the field to just 24 players for a final day consisting of six-hole match-play ties.
In Adelaide, a bogey-free 65 helped English player Jodi Ewart Shadoff to a share of the lead after the first round of the LPGA-sanctioned Women’s Australian Open.
The Solheim Cup contender made six birdies in her first ten holes to set up the seven-under effort in perfect early-morning conditions at The Grange.
Solheim Cup captain Catriona Matthew was the best of the Scottish players, shooting a one-over 73 in breezy afternoon conditions.
In South Africa, Bradley Neil opened with a one-over-par 73 in the Dimension Data Pro-Am at Fancourt to sit 10 shots off the lead, held by home player Riekus Nortje, with another South African, Justin Walters, second on eight-under.
Meanwhile, the R&A and the USGA have issued a “periodic update of progress made to date on the Distance Insights project”, revealing that delivery of report is anticipated later this year.