The whole issue of Lawful isn't a problem whilst a State of Emergency exists, once that no longer applies all those health directives are no longer relevant.
Hence why the government is leaving it to businesses to fight the mandate. However the CHO still has powers to direct individuals to isolate and test etc via individual notice. The SOE only provides for general overall directives of the overall public, restriction of movement on a mass scale. The CHO can still tell the individual that has or has contacted someone with covid to isolate and restrict their movement outside of SOE.
In an Airlines case, it's passengers. Not all business's can use a flight attendant to check as you pass through the boarding process 3 times.
Airlines are moving to mandate only staff be vaccinated, pretty sure like allergy control they won't be doing much to check passengers.