McDade was first reserve and learned he was in the field only minutes before teeing off at 7.38am on Thursday after the PNG PM pulled out due to his official duties.
Unfazed, the former Australian amateur representative proceeded to shoot a five-under 67 at Royal Port Moresby Golf Club and he shares the lead with fellow young gun Josh Greer (WA), William Bruyeres (Qld) and in-form Brett Rankin (Qld) after day one.
"I mean, this is probably as good as this day could have gone for me," said McDade.
"There was a lot of uncertainty, but yeah, played really well and took it how it came so I'm happy."
Marape, an avid golfer, had played in his national Open in 2023 and had been set to do so again, but McDade was grateful for the opportunity presented.
"I came a long way to come and play golf and was just waiting to see what happened for me," he said.
"It was definitely an easier day than I thought it was going to be. I was kind of prepared to sit on the first and 10th tees for 12 hours, but it didn't really work out that way.
"I was off the tee within an hour, so yeah, that was really lucky."
Five players - James Gibellini (Vic), Zachary Maxwell (Qld), Andrew Campbell (NSW), James Hydes (NZ), and Andre Lautee (Vic) - share fifth position, just one back from the leaders.
The leading PNG player after day one is amateur Morgan Annato, who carded a tidy 73.
