There is a prominent rock pinnacle 800m east of Mt Petermann, at about 2260m. From the upper Amherst Glacier, climb snow slopes between Petermann and the rock pinnacle, leading to a snow ridge that leads past the pinnacle to the summit of Mt Shyness.
Elaine Norton and J Shanks climbed a virgin peak in this area in January 1941, which may have been the pinnacle described above.
Alternatively, climb the snow face of Petermann from the Amherst, then over the ridge to easy snow slopes on the Sealy Pass side.