Richard Kevin Thirkell, of Newcastle, was a roof plumber on an Abdon Close worksite when Edgeworth mother Judith Davis, 41, fell 9.6 metres through a roof to her death on April 25, 2003.

Thirkell, 37, originally pleaded not guilty to manslaughter. He changed his plea yesterday in Newcastle District Court.

Thirkell's then-employer, David Peter Smith, 53, pleaded guilty to the charge on Tuesday.

The cases are understood to be the first ever successful prosecutions of industrial manslaughter in NSW.

The court has heard Smith's business was contracted to strip and re-sheet the roof of an industrial building.

Ms Davis, who had been in a long-term relationship with Smith and was helping at the site, died after she fell though a section of partially secured polycarbonate roofing and wire safety mesh, which gave way when she walked across the roof. The safety mesh had not been attached to the roof in one of two approved ways.

There was no clear evidence as to who secured the mesh at that section, but it was not Smith, the court heard. He pleaded guilty on the basis of his failure to ensure the work was carried out properly.

Thirkell pleaded guilty on the basis that, as one of the more experienced employees, he also failed to ensure the wire mesh was attached properly.

In the witness box yesterday, Thirkell said he tried to grab Ms Davis as she fell, but "missed her hands".

"This day will never leave me . . . it will never go," he said.

The case was adjourned to today.