Macrocell corrosion with a local anode and a large cathode frequently occurs in chloride induced corrosion of rebars in concrete and is responsible for very high local corrosion attacks and reduction in cross-section found e.g., in bridge decks or substructures. In model-macrocells in electrolytes and in mortar the influence of the conductivity and cover depth on potential and macrocell current distribution have been studied both in open circuit conditions and under external anodic polarisation. The results have shown that low electrolyte conductivity and low cover facilitate the location of the anode of the macrocell by potential measurements due to a concentration of the spreadout of the macrocell action. Under anodic polarisation the imposed current is concentrated on the local anode. The consequences for corrosion monitoring by half-cell potential mapping and by polarisation resistance measurements on locally corroding rebars are discussed.