The article present an ultralow frequency passive vibration isolation device as part of the preisolation stage for the Australian International Gravitational Observatory (AIGO). This isolator is based on the Roberts linkage and simulates a very long radius conical pendulum to provide two-dimensional isolation. It is designed as the second horizontal isolation component of the preisolator and it will support the remaining four isolation stages of the AIGO vibration isolator chain. A description of the device together with a theoretical model and its measured isolation performance are presented. With the current setup, we obtained 32 dB of isolation at 1 Hz. (C) 2003 American Institute of Physics.