I installed the free mesh for Australia provided by ORBX but for some reason this occurs across Australia.

 

Any resolution would be great :D

I believe you are talking about the "Holgermesh"?

 

If so, ensure that the order (priority) you place it with your Orbx suite of sceneries, is above all Orbx stuff but under (lower) than IE in your scenery library view inside of FSX the order should be......and then everything else should be below ie the regional sceneries and individual airports.

 

 

ORBX\FTX_AU\FTXAA_AIRPORTS

ORBX\FTX_AU\FTXAA_HOLGERMESH

ORBX\FTX_AU\FTXAA_ORBXLIBS