That would most likely be either :-
G28 G91 Z0
or could be
G53 Z0
Depending on your machine will send the CNC to your Z0 Home position, which version of the Mach3 Post Processor are you using and where are you storing it, if you leave it in the "Installed Library" in Fusion then it will be automatically overwritten at a Fusion update, should be kept either in the "Cloud" or in your "Local Library".
Put your Post Processor in a Zip file and upload to here and someone will have a look at it for you.
Do you have a sample of nc code that is good for comparison << If so can you upload that as well