The 30 minute figure seems pretty arbitrary. Some aircraft I've flown if that was all that remained in the tank would be a worry. Just a simple rate one turn could be sufficient to slosh the fuel away from the tank outlet hole causing surging or loss of supply.
In an Aero Commander it has one collector tank aft with the outlet hole at the very bottom.
But in the case of yesterday's Bankstown incident, if it was fuel starvation, a rate one turn with very limited fuel may have uncovered the out take hole.
CASA No matter what they dictate will never stop idiots running out of fuel. All they will do is chase reasonable people to prove their mantra of "Safe Flying"