Arnold was expected to be locked in a scrap to start with Lyon goalkeeper Teagan Micah, who last week withdrew from the squad after another concussion.
Given the two other goalkeepers are the untested Jada Whyman and three-cap youngster Chloe Lincoln, Arnold is a nailed-on starter for the whole tournament.
Arnold, who turned 32 on Wednesday, plays for NWSL club Portland Thorns, who are currently in pre-season.
But she played down her relative lack of competitive minutes ahead of Sunday's opener against the Philippines at Perth's Optus Stadium.
"Obviously not ideal having the off-season over the Christmas break," Arnold told reporters in Perth.
"But again, we've been fully in pre-season now. Had two pre-season competitions as well, so plenty of game time in that.
"So yeah, it's been good."
Arnold insisted she didn't need to have the surety of knowing she was the first-choice goalkeeper in order to perform.
"Not really. If you even look at the World Cup that was my first year starting as the starting goalkeeper, and you've just got to take each game as it comes," Arnold said.
"I don't think your position's ever a certain spot, unless maybe if you're Sam Kerr.
"But it's very different obviously with the goalkeeping position, we've always been in very heavy competition and we've known that.
"... I, especially, take each game as it comes, I never get too far ahead of myself."
Expectations of the Matildas under Joe Montemurro are high ahead of the tournament on home soil, but Arnold insisted they weren't feeling the pressure.
"I wouldn't say pressure. I think there's a lot of expectations," Arnold said.
"There's always going to be pressure surrounding a home tournament, but I think we sort of try and keep that internal.
"We know what we can do, we know what we expect of each other. I think after coming off the Olympics, it probably wasn't our most successful campaign, and we know that.
"So yeah, I definitely think there's expectations, but in terms of pressure we just sort of block that out and just keep it internal for us."
