A guide device (10) is disclosed for constructing a wire or cable fence. The device (10) is arranged for contact with, and movement along, strands of pretensioned wire or cable (64) which form the fence wires. The guide device (10) has a number of rotatable members (16) on a generally horizontal axis of rotation, which are each arranged in use to support one of the strands (64) at a respective, pre-determined elevation above surrounding ground (8). A plurality of fence posts (66) having through-holes (68) that are arranged for receipt of the respective strand(s) (64) can then be caused to slide along the strands (64) in use without contacting the surrounding ground (8), and be spaced apart along those strands (64) to form the posts of the fence.