We report the discovery of two new large H i shells in the direction of the Coalsack nebula .
Both shells were observed with the Parkes Radio Telescope as part of the Southern Galactic Plane Survey .
The largest shell , GSH 304-00-12 , is at a distance of \sim 1.2 kpc and has derived physical dimensions of 280 \times 200 pc .
The second shell , GSH 305+01-24 , is at a distance of \sim 2.2 kpc and has derived dimensions of 280 \times 440 pc .
We present a simple numerical model to show that GSH 305+01-24 most likely formed from stellar winds in the Centaurus OB1 stellar association .
There is associated radio , infrared and H \alpha continuum emission .
Both shells are situated in the Sagittarius-Carina arm , with GSH 305+01-24 more distant .
The far edge of GSH 304-00-12 is at the near side of the arm and opens into the interarm region .
We find no evidence for closure at the near side of the shell and therefore describe the geometry as conical .
Emission from the near side of the shell may be lost in absorption by the Coalsack Nebula .