There is nothing written therefore there is no requirement for notice of lockout.
So why then is there so much mention of the '72 hours notice' in the FWA hearing? The entire Qantas argument for the shutdown of the airline was that they had to give 72 hours notice and that it was a safety threat so therefore it is safer to ground the airline. If there is no requirement as you say the statements in the hearing are completely illogical. Their whole justification for the shutdown was that they had to give 72 hours notice.
If QF could have just locked out the workforce as you say why didn't they just do that rather than stuff around with a safety based shutdown?