richard_css , my reply has errors, here is the corrected text (I am unable to edit my post now):
this is an ongoing issue for users. Invoices posted from external systems are processed through the same 'engine' as if they were entered manually. AccountRight takes the supplied line value (as net or inclusive depending on the isTaxInclusive flag, I think) and calculates the GST. (The line GST amount is not provided in the content when posting invoices).
The second issue is that AccountRight ensures, by adjusting the calculated GST line amount, that the GST calculated on the taxable total so far is correct. This is why the OP sees random varying net line amounts.