US RETAILER Costco is seeking to break into Australia's $15 billion-plus discount petrol business, currently dominated by Woolworths and Coles, by selling its own heavily reduced fuel at prices below those offered via popular supermarket shopper-docket schemes. Costco, which operates a warehouse store in each of Melbourne, Sydney and Canberra, has requested room to build petrol stations at a second store slated for Sydney and for its first in Brisbane, and is awaiting council approval on both. If the petrol outlets are approved, it will be the first time Costco can offer petrol to its local members along with its range of drastically discounted food, liquor, clothing and electrical products that has made its warehouse-style outlets so popular since they arrived in Australia three years ago. ''If you can sell vegemite at a cheaper price on a full pallet, or a kilo tub, why can't we sell gasoline?'' Costco's Australian managing director, P...