By the level of interest generated in last light calculations maybe a separate topic is warranted where definitions from various jurisdictions can be compared & debated?
On the subject of the accident in question, at the time the aircraft was reported missing I was talking to a friend on the phone who lives in Newcastle. He told me that there was a search underway so I opened the BOM website radar & had a look, there was a definite band of heavy showers oriented approximately North/South moving out to sea off the coast of Newcastle.
Even if it was still "daylight" (which it wasn't), over water in showers with fading or no light would have made it difficult to remain VFR, as mentioned several times previously. The reasons behind the decision to turn left instead of land may never be known, but I hope that the "holes in the cheese" which lined up & led to this decision are determined as it might help the rest of us avoid a similar fate.