There's no need to call someone in when a post or rail needs renewing.

To repair a damaged post
After prolonged exposure to moisture in the ground, the bottom section of fence posts may become damaged by rot or termite attack. To replace this lower portion of the post, carry out the following steps.

1. Remove the palings from the area to be repaired. Support the fence by bracing it.

2. Cut the damaged section of the post away. Dig out the old footing and the damaged post. (It is often difficult to remove posts from concrete footings. Use levers and a fulcrum to ease the post out rather than digging a larger hole which will require increased quantities of concrete to refill.

3. Prepare the new section of the post from creosoted hardwood or treated pine. Cut the new length to go into the ground.

4. Brace either side and put two galvanised bolts through the braces and old section of the post. Put two galvanised bolts through the braces and new section of the post. Concrete in position.

5. Replace the fence covering.

To repair damaged rails
To replace an entire rail, remove the palings from the panel of the fence. To repair a section of rail, carry out the following steps.
6. Remove the palings from the area to be repaired.

7. Prepare a 200mm long lap or notch joint to connect the new rail section to the existing section. Alternatively, use metal brackets or plates to connect the notched rails.

8. Fix into position using coach screws. Replace the palings.

Source: Fences (Murdoch Books) (out of print)