On a day when some of the game's bigger names fell apart, Pettersen displayed the talent that won last year's French Open. The Norwegian was Europe's rookie of the year.
Her second straight 70 included birdies at the 12th and 16th holes, while Webb, after an opening-round 68, slipped to 72 Friday. Both have two-round totals of 4-under-par 140.
Bae Kyung-eun of South Korea and Australian amateur Nikki Campbell were two shots back at 6-under 142.
Claiming she was the victim of a 'typical Melbourne day,' Webb's scrambling round took its toll when she bogeyed the 17th to drop into a tie with Pettersen. Webb then missed a 7-foot birdie putt on the 18th.
'It was just one of those days when it was cold, it rained, it got warm, it blew,' Webb said.
Webb, who led after the first round, lost in a four-hole playoff last week to Annika Sorenstam at the Australian Ladies Masters.
While there was little movement at the top, there was plenty at the bottom.
Former world No.1 Laura Davies, who finished with an eagle-3 on her last hole Thursday, floundered to a 9-over 81 Friday. She had a triple-bogey, two double-bogeys, two bogeys and no birdies.
Australian veteran Jan Stephenson added a 79 to her opening 72, and mother-to-be Karina Orum ended the day 16-over after a 79 Friday.
The event is co-sanctioned on the European and Australian tours.