In the sweltering conditions, which saw the teams stop for water breaks in both halves, the Ferns dominated throughout.
In the 53rd minute Longo scored a fine individual goal - dribbling past several defenders before slotting home - then captain Abby Erceg made it six 10 minutes later. Hearn, in her 105th international, volleyed a second in the 64th minute. The hosts finally got on the scoreboard in the 79th minute through Meagan Gunemba.
The second leg of the Oceania Football Confederation playoff is at Albany Stadium on Tuesday evening.
Readings was content with Saturday's effort which provides a good buffer for the side heading back to Auckland.
"We wanted to come back to the second leg with a win and if we could come back with a comfortable lead like we have, then we'd obviously be really happy," Readings said.
"PNG did make it hard for us, they worked hard all the time. They were very aggressive and very physical. But the good thing is we just kept going.
"It would have been easy when we were 4-0 up at halftime to take our foot off the pedal but we kept pushing forward and kept trying to do the right things all the way to the end. I'm really happy with the performance and the result."