Killing Ground (2016) on Netflix

Plot summary

Ian Smith and Samantha Shaw, known as Sam, take a romantic trip for New Year's Eve to a remote outback beach, away from the pressures of city life. On the way they meet German, who points them toward a beach by a waterfall. They pitch their tent beside a campsite that is empty, with no sign of whoever stayed there. Sam suggests that she and Ian marry, and he agrees. After a few nights they discover a traumatized baby on his own. German tells his associate Chook about the couple, and the two start planning.