You are primarily correct max, all major fuels do come from the BP refinery & Kewdale terminals but they don't clean out the tanks at the stations. Rarely they may pump out the water which is heavier than fuel and sits at the bottom of the tanks with the sediments. Each station has alarms for each tank that are set to go off when the tanks get down to 5000L (can't remember exact amount, may be more). The problems with contamination arise when the attendants ignore the alarms and let the tank drain to the sediment level. The somewhat expensive pumps not only fill your tank with shite but also fail within a very short time frame. Also avoid filling up at a station that is doing the same, sediments get stirred up by the tankers.
BP/Shell premiums are actually about as good as it gets. The independants just water it down so to speak.
Courtesy of a conversation about fuels I had with a state manager for BP my brother in law, so it's not BS.