In a medium saucepan, cook potato in water for about 12 minutes, or until tender. Do not drain. Cool mixture to 43 degrees C. Set aside 1/2 cup of the cooking liquid. Mash potato and remaining water adding warm water to make 2 cups of potato mixture.
In a large mixing bowl, sprinkle yeast on reserved potato water. Add mashed potato mixture, 2/3 of the flour, sugar, butter and salt. Beat with an electric mixer on low speed for a minute, scraping sides of bowl constantly. Beat 3 minutes at high speed. Stir in as much of the remaining flour as you can, mixing in by hand.
Turn dough out onto a lightly floured surface. Knead in enough of the remaining flour to make a moderately stiff dough. Knead for 6 to 8 minutes, or until dough is smooth and elastic. Place in a lightly oiled bowl; turn once to coat surface. Cover. Let rise in warm place until doubled in bulk, about 1 hour.
Punch down, and turn out onto a lightly floured surface. Divide dough in half. Cover, and let rest for 10 minutes. Shape each half into a loaf. Place in greased loaf tins. Cover. Let rise until nearly double, about 35 minutes. Before baking, brush tops with a little water and dust with additional flour.
Bake at 190 degrees C for 40 to 45 minutes. Cover with foil for the last 15 minutes of baking to prevent over-browning. Remove from tins; cool on wire rack.