The second goal came a minute before the breather when lanky teenage striker Angus Kilkolly used his pace and stride to toe a ball through, prompting Marist centreback Fin Milne to bring him down from behind.
Referee David Beattie pointed to the penalty mark and Matt Hastings planted it past Singh for a 2-0 lead.
Marist coach Simon Lees said it was definitely a penalty but there was a bit of commotion when the ref returned to the net after the ball had snuck right through.
"I'm not questioning the goal. It went in because everyone was running back to the centre," Lees said but a ref's assistant informed them he had checked the net and tied up a hole before the kick off.
The hosts got a consolation goal from Jonothan Steele one minute into the ref's time after captain Danny Wilson and his team of defenders were caught napping.
Woody Maxey-Graham put a deft cross into the box but the Rovers let it curl to an unmarked Steele who nodded the ball into a gaping net as goalkeeper Jonty Underhill came out to claim the ball.
"It was some poor marking from us. I think we thought the game was over," Robertson said as the Blues maintained their third spot on the ladder.
Lees said in many respects conceding the penalty summed up Marist's game.
"We didn't defend well but individually we were not bad.
"Napier City made us commit as a team and we didn't do it," he said, lauding the visitors for executing a well-thought plan.
Marist missed striker Tomas Mosquera (bereavement) but are hoping he'll be there in Napier on August 3.
The Rovers host Petone FC at Bluewater Stadium, Park Island, this Saturday in a 2pm kick off before hosting Wellington Olympic on Queen's Birthday Monday in the second round of the Chatham Cup (national knockout cup) match.
