Depends on which sources you believe, but apparently Whitlam sounded Hawke out about taking the leadership directly after the disastrous '75 election defeat; Hayden and Clyde Cameron, quite separately, 'offered' the leadership to Don Dunstan; and I have a feeling Cameron also spoke to Hawke to 'offer' him the leadership at around that time.
Of course none of the aforementioned MHRs actually had the power to hand the leadership of caucus to an outsider, even if said outsider was a political superstar who was able to parachute into the reps in a bye-election for a safe Labor seat (and really, it's not like there was any happy precedent for that happening in federal politics--see division of Dalley, 1927). So these supposed offers were all just idle political gestures.
Ultimately I think Hawke timed it pretty well. He showed uncharacteristic restraint in not going all out to get a seat at the election of 1977--and it's not as if he had any plans to end his career as a mere ACTU president.