Is it illegal for a supplier to charge me a different price from that which he charges my competitor?

Not necessarily. Different prices may simply reflect variations in the costs of supplying different customers (higher transport costs for example) or may reflect discounts for bulk purchases. Price discrimination may, however, be an infringement of the Sections 17 or 20 of the Act where there is evidence that the prices charged are excessive or are used to lessen competition substantially (e.g. because they are predatory or are designed to foreclose markets). Of course, in these circumstances it would have to be proved that the practice is having or is likely to have the effect of substantial lessening of competition. In the case of Section 20, the supplier must be shown to hold a dominant market position.