The "decision maker" was technically correct in that an Australian passport holder does need a visa to enter Lebanon, but that visa is normally issued on arrival, which may be the cause of the confusion. IIRC, BA uses a version of timatec to check visa requirements and for Australian passport and Lebanon it says:

Document validity rules:

Passports and other documents accepted for entry must be valid for a minimum of 1 month beyond the period of intended stay.

Minors under 18 years of age can enter with passports valid for the period of intended stay, when accompanied by a parent.

Important:

Holders of passports containing any Israeli visa or stamp will be refused entry.

Maids must ensure that their passport includes a departure stamp issued by the country they departed from.


Lebanon - Destination Visa

Visa required.

Visa issuance:

Nationals of Australia traveling as tourists can obtain a visa on arrival at Beirut (BEY) for a maximum stay of 1 month. They must have a telephone number and address in Lebanon. They can apply to extend their stay for an additional 2 months.

Passengers with a confirmation that a visa has been approved before departure issued to businessmen, bankers, directors and investors by the Lebanese Immigration Directorate can obtain a visa on arrival at Beirut (BEY) for a maximum stay of 6 months.

Tourist groups of minimum 8 people can obtain a visa on arrival at Beirut (BEY) for a maximum stay of 6 months. They must be sponsored by a registered tour operator in Lebanon.

Passengers with a confirmation that a visa has been approved before departure issued by the Lebanese Immigration Directorate can obtain a visa on arrival at Beirut (BEY).

Maids of passengers who are accredited to Lebanon can obtain a visa on arrival.