The vicinity of New Zealand/Australia is known to cause problems with GPS fix when SBAS is enabled.
For our vendors in that part of the world we supply AIS with SBAS disabled for that reason, but generally SBAS is a reliable and beneficial enhancement to precision, so is enabled by default.
If a customer complains of "position drift" or "GPS inaccuracy" You can try instructing the customer how to disable SBAS manually. It can be done by sending the following sentence from the proAIS2 Serial Data tab:
$PSMT,0,0,0,1,setsbasmode 0,0*59
You can then verify that SBAS has been disabled by checking the GNSS status tab in proAIS2. When SBAS is deactivated, you should see only Green and Blue bars on the chart.
If the customer wishes to restore SBAS (I suggest not in the South Pacific) then the following configuration will do that:
$PSMT,0,0,0,1,setsbasmode 1,0*58
However it may be simplest to leave it disabled. At sea with a good population of visible satellites, uncorrected GPS is normally sufficient for AIS operation, and is explicitly prohibited for Class A in any case.