The 20 May 2016 surface‐rupturing intraplate earthquake in the Petermann Ranges is the largest onshore earthquake to occur in the Australian continent in 19 yr. We use in situ and Interferometric Synthetic Aperture Radar surface observations, aftershock distribution, and the fitting of P‐wave source spectra to determine source properties of the Petermann earthquake. Surface observations reveal a 21‐km‐long surface rupture trace () with heterogeneous vertical displacements (). Aftershock arrays suggest a triangular‐shaped rupture plane () that intersects the subsurface projection of the major geophysical structure (Woodroffe thrust [WT]) proximal to the preferred location of the mainshock hypocenter, suggesting the mainshock nucleated at a fault junction. Footwall seismicity includes apparent southwest‐dipping Riedel‐type alignments, including possible activation of the deep segment of the WT. We estimate a moment magnitude () of 6.0 and a corner frequency () of 0.2 Hz, respectively, from spectral fitting of source spectra in the 0.02–2 Hz frequency band. These translate into a fault area of and an average slip of 0.36 m. The estimated stress drop of 2.2 MPa is low for an intraplate earthquake; we attribute this to low‐frictional slip (effective coefficient of friction ) along rupture‐parallel phyllosilicate‐rich surfaces within the host rock fabric with possible additional contributions from elevated pore‐fluid pressures.