"Rhema" in Greek literally means "utterance" or "thing said". In Christianity it is used in reference to the concept of Rhematos Christou or Jesus Christ's sayings. Radio Rhema began in 1974 in Christchurch, New Zealand although its roots go back to the 1960s when they were known as Gospel Radio Fellowship. After multiple temporary licenses to broadcast (sometimes for one day only) they received a permanent license in 1978. This made them the first Christian radio station in the British Commonwealth and one of the first in the world. In the 1980s, they branched out to Australia which is where most of these Rhema stickers are from.
UPDATE: 612 AM is in Christchurch and 855 is in Hamilton. Thanks for the info Anonymous commenter!