Should your existing timber post become rotten and broken off at ground level but still has a firm concrete base, you can take advantage of the metal repair spur. It can be tapped into the remaining timber to accommodate a new timber being placed in the top of the repair spur, thus eliminating the need for the post to be excavated out from the ground. 
 
Metal repair spurs are available to purchase for 3” x 3” and 4” x 4” timber posts.