The original reason for the twelve hour shift was because almost everyone was collecting four hours of overtime, and the taxman was then taking at least half of that extra money. This in effect ment that people were working 60 hour weeks with only two days off. LAMES at Ansett at least had decided by 1980 that it just wasn't worth it.
They put the 12 hour idea to me back in about 1980, it looked like a win/win to us, the only loser being the tax man. I dutifully carried it up the tree, but Abeles made it quite clear that he wasn't going to be the one to break the sanctity of the "Eight hour Day" for obvious reasons and the proposal died.
I would suggest that the Eight hour shift is all about coverage and flexibility for management, not about money. You are "at the office" an extra day and are therefore available to work extra overtime if required, thus allowing the management to get by with less staff by working you harder....................
...Or something like that, you guys no more about the details than I do.