z axis goes to Machine 0 and not soft 0

[nava]

I'm having a little problem with setting my Z axis.  When I zero out my work piece to start a job Mach uses machine 0 on the Z axis instead of the soft 0. The machine will zero to the soft X and Y but not Z and the only thing that I have found to fix this is to first 0 out all the axis then do a restart of mach3.  Ive looked through the settings and so far cant see whats causing this  and ideas?
Here is an example of the files im loading...just the start of it anyway
%
O5000 (12MM_TUBE_HOLDER_CORRECTED.TAP)
( MCV-OP ) (02-NOV-2011)
(SUBROUTINES: O2 .. )         
G90 G17
G80 G49 G40
G54
G91 G28 Z0 //When the machine reaches here its at the machine z and not soft z
G90
M01
N1 M6 T1
(TOOL -1- MILL DIA 6.0 R0. MM )
G90 G00 G40 G54
G43 H1 D31 G0 X16.897 Y20. Z50. S1000 M3
M8

[Hood]

Have a look at the description of G28 in the G Code list.

Hood

[nava]

thanks hood worked it out its all todo ith the post file im using and understand the g28 now:)