As a child in west Cornwall, in the 1930s, I knew perfectly well the meaning of a "Methodist wowser". As it was widely used among the older generation I imagine it is a 19th century word, or earlier. With so many Cornish emigrating to Australia then, no need to ask how "wowser" became part of Australia's vocabulary, along with many other words of Cornish origin.
The meaning "killjoy" is an obvious derivation from a the word's use to describe someone who would forbid not only newspapers but games or music on a Sunday and alcohol (in Australia!) on any day of the week.
Halifax, West Yorkshire