Volcanoes in the Great Barrier Reef
Australia's Institute of Marine Science webpage How the Great Barrier Reef was formed is the result of the flooding of the former coastal plain when sea levels were lower.  A further explanation from the website states:


  The coastal plain included river valleys and small and large hills. Corals tended to grow best on hills because they were where waves broke and the water was most turbulent. Thus, although corals probably grew on any available hard shallow surface on the flooding coastal plain, reefs tended to form where there were hills.


A diagram of the formation is shown below:



Image source

So, not the result of volcanoes, but rather the after effects of coastal plain inundation.