Welcome Patto, I think Gyprock have a DIY sheet called "covering a doorway".
The steps:
remove architraves and door jamb. This will expose the studs on either side of the opening. They will be either 70 or 90mm wide. Nail some similar studs to the jamb studs and one in the middle of the door. Fix a bottom plate to the floor with nails. Line with gyprock and tape and finish with plastering basecoat and topcoat.
Cheers
Pulse