Ryan Tinsley pushed the Danny Wilson-captained Rovers 2-1 up in the 61st minute but five minutes later Khair Jones levelled terms again.
It took until almost five minutes from regulation time before golden boot leader Angus Kilkolly broke the deadlock with a penalty kick after defender Ashish Sahayam "blatantly handled" the ball in the box.
Rovers midfielder Josh Stevenson put Marist out their misery, 4-2, in the 89th minute.
"We put them under pressure all game but it was a tense second half and we conceded some sloppy goals," Robertson said, lamenting his players missing about six chances, whereas Marist had two and scored from both.
The result was a fair result for the coach who impressed on the Rovers at halftime the need to continue as they had been - bar the last five minutes when Marist scored a breakaway goal.
"I asked them to apply more pressure but we did that only in patches."
The Rovers, with one league loss this winter, host Lower Hutt at 2pm on Sunday and morale is high.
"We're confident we can beat anyone," Robertson said.