Ok your domain is slightly smaller than the default Lismore outer domain which is 450x450.
Try reducing your number of processors, say 16x12. So set
rg01_rs01_m01_nproc=16,12
in rose-suite.conf. Or you can use rose edit and change the values in the GUI.
Remember to reload your suite. You will have to run the _GAL9_LBCS and _GAL9_um_recon tasks again with the new decomposition before running the forecast tasks again.
Generally when running on the normal compute nodes on gadi (which have 48 cores) we want the total number of cores to be a multiple of 48.
When the UM decomposes your grid across multiple cores, you need to reserve a certain number of grid rows for the halo - the points that are shared between each sub-domain. The UM has limits on the numbers of points in the halo region and the sub-domain itself.