The Grasstree is located on the north-western flank of the Bowen Basin, a sedimentary basin comprising Permian to Triassic age geology. A veneer of more recent Tertiary and Quaternary age sediments typically overlie the Bowen Basin strata.

The relevant geology in the vicinity of the Grasstree longwall mining area includes:
- Quaternary alluvium associated with German Creek and Little Parrot Creek;
- A veneer of Tertiary sediments of the Duaringa Formation;
- Permian coal measures including the target German Creek Seam (GC seam).

The Permian coal measures include the Rangal Coal Measures, the Burngrove Formation, the Fort Cooper Coal Measures (also known as the Fairhill Formation), the Macmillan Formation and the German Creek Formation. The coal measures comprise a sedimentary sequence with interbedded coal seams, including the target GC seam. The upper profile of the coal measures has been extensively and deeply weathered. The weathered coal measures outcrop at the surface across the southern portion of the longwall mining area and much of the surrounding area, forming undulating sandy plains. In the northern portion of the longwall mining area, the coal measures sub-crop under the Tertiary sediments and dip to the east. As the coal measures dip to the east, the depth of the GC seam increases from approximately 330 m in the south-western part of the Grasstree longwall mining area to a maximum depth of approximately 540 m at the north-eastern extent of the longwall mining of area.

A longwall is a complex system of mining equipment that incorporates hydraulic roof supports (called ‘chocks’ or ‘shields’), coal cutting and coal transport equipment. Longwall mining involves extracting rectangular panels of coal, typically around 150 m to 400 m wide, up to 7 km long and 2 m to 5 m thick. Longwall panels are defined by access roadways that are constructed around the perimeter of each longwall panel. These roadways provide access for the installation of the longwall mining equipment, mine workers, and equipment and services.

Longwall mining involves a coal shearer travelling back and forth across the width of the longwall panel, starting from the furthest point progressively removing the coal from the panel back to the main headings. The shearer cuts the coal from the coalface on each pass and delivers the coal to a face conveyor that runs along the full width of the longwall. The face conveyor transports the coal from the coalface to another conveyor in an access roadway. Coal is then transported to the surface via a series of connecting underground conveyors.

The roof at the coalface is held up by a series of hydraulic roof supports. The supported section of roof provides space for the shearer, face conveyor and man access. After each shear of coal is removed, the face conveyor, hydraulic roof supports and the shearer are moved forward.

The roof immediately above the mined seam collapses into the void (called a ‘goaf’) that is left as the roof supports progressively retreat through the panel. As the roof material collapses into the goaf behind the roof supports, the fracturing and settlement of the rocks progresses through the overlying strata and results in the sagging and bending of the near surface rocks. This can result in the progressive formation of gentle trough-like depressions on the surface relative to the natural topography (called subsidence). The subsidence effect moves across the ground at approximately the same speed as the advance of the mining face, which is typically up to 100 m per week. The majority of subsidence occurs on the surface within three months of undermining and all subsidence is generally completed within 12 months.

Longwall mining will be undertaken in the German Creek seam. This is the same seam being currently mined at Grasstree Mine. The longwall panels will be approximately 340 m wide and will range from approximately 2.4 km to 3.3 km long. The extraction height will vary from 2.4 m to 2.6 m. The width of the proposed chain pillars (the coal left between the longwall panels) will be approximately 50 m. The target coal seam in the project longwall mining area is at a depth of between 330 m and 540 m.

Coal from the underground workings the Grasstree Mine is transferred to the surface via the Southern Colliery drift conveyor. The conveyor drift portal is located in the highwall of Pit A. The drift conveyor travels from the portal, up the Pit A ramp, to the Grasstree Run-Of-Mine (ROM) coal stockpile. Coal from the ROM coal stockpile is transferred through a breaker station, where it is sized and coarse rejects are removed. The coal is then loaded onto an overland conveyor for transport to the CHPP. The coarse rejects are trucked from the breaker station and placed in the Pit B open cut void.

ROM coal is washed at the CHPP. Product coal is transported from German Creek mining complex via an onsite rail loop and train loading facility. Product coal is railed to Central Queensland ports, including Dalrymple Bay Coal Terminal, for export

Rejects and tailings produced from the washing of Grasstree coal at the CHPP are stored in the German Creek inpit rejects and tailings storage areas. Excess mine water from the dewatering of the Grasstree Mine underground workings is transferred to the German Creek mine water management system.