Bob, this is the method copied from the Austpost website. How do I get self-adhesive stamps off an envelope?
Self-adhesive stamps have two layers of adhesive. One is the tack that bonds the stamps to the envelope; the other is a water-soluble adhesive that lies between the self adhesive tack and the stamp paper. The water-soluble layer is there so that collectors can obtain good used copies. (very thoughtful of them)
Self-adhesive tack is practically impervious to water on one side of the stamp and the varnish creates a waterproof seal on the other. When soaking by conventional methods water cannot reach the water-soluble layer effectively.
Although several substances have been touted as means of overcoming the problem, we do not favour the use of anything other than clean water. To give the water a chance to activate the soluble layer, it needs to be almost at boiling point and the stamps need to be immersed for up to half an hour. The stamps should then be gently eased off the paper and, when dry, any remaining tack removed carefully with an art gum eraser.
(I find it doesn't take as long as they mention)