Boulden, 23, struck six birdies to card a six-under-par 68 after Australian Sarah Jane Smith took the lead with 67.
In a close contest, with the top 19 separated by just four shots, England's Jodi Ewar-Shadoff hit 71 and is four strokes back.
Scotland's Gemma Dryburgh and Michele Thomson are one shot behind Ewar-Shadoff after rounds of 69 and 74.
Wales' Amy Boulden is three shots off the lead after the second round of the Women's Australian Open in Adelaide.