Whilst this might seem obvious to everyone except me, it’s taken me ages to find this out! This assumes you already have compass installed and useable from the command line. To compile SASS in a Netbeans (I’m using v7.4) project, go to File > Project Properties > CSS Preprocessors. Tick ‘Compile Sass Files on Save’ and adjust the input and output directories to suit your project.
Here comes the magic bit! Type
into the compiler options to compile using Compass instead of the default SASS compiler.