His work uses photography and video. His best known work is Bullet Hole which is a closeup photo of what appears to be a bullet hole wound in the scalp of a person's head, mounted on 15 light boxes. Collishaw took the original image from a pathology textbook that actually showed a wound caused by an ice pick.Bullet Hole is now in the collection of the Museum of Old and New Art in Hobart, Australia.
In 2012, to go alongside the AKA Peace Exhibition at the ICAArt Below showcased selected works from the AKA Peace series on the London Underground including Collishaw's. "AKA Peace" originally conceived by photographer Bran Symondson and now curated by artist Jake Chapman, is an exhibition of new works made specially for The Peace One Day Project 2012, bringing together a group of Contemporary Artists, all of whom agreed to transform a decommissioned AK-47 assault rifle, refashioning into artworks.
British pathologist, Austin Gresham, wrote a handbook, A Colour Atlas of Forensic Pathology, in 1975. Collishaw said it became "the Britart bible", as a source for explicit images of dead bodies for artwork.