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No way they were 100 tons overweight

Those might have been metric tons, as in 'tonnes' but the figure was 100 ... per:

www.atsb.gov.au/publications/investigation_reports/2009/AAIR/pdf/AO2009012_Prelim.pdf

Tail Strike
Melbourne Airport, Vic.
20 March 2009
A6-ERG
Airbus A340-500

Abstract
At 2231 Eastern Daylight-saving Time, an Airbus A340-500 aircraft, registered A6-ERG, commenced the take-off roll on runway 16 at Melbourne Airport on a scheduled, passenger flight to Dubai, United Arab Emirates with 257 passengers, 14 cabin crew and four flight crew.
The takeoff was planned as a reduced-power takeoff and the first officer was the handling pilot for the departure. At 2231:53, the captain called for the first officer to rotate.
The first officer attempted to rotate the aircraft, but it did not respond immediately with a nose-up pitch. The captain again called ‘rotate’ and the first officer applied a greater nose-up command.
The nose of the aircraft was raised and the tail made contact with the runway surface, but the aircraft did not begin to climb. The captain then selected TOGA on the thrust levers, the engines responded immediately, and the aircraft commenced a climb. The crew notified air traffic control of the tail strike and that they would be returning to Melbourne.
While reviewing the aircraft’s performance documentation in preparation for landing, the crew noticed that a take-off weight, which was 100 tonnes below the actual take-off weight of the aircraft, had inadvertently been used when completing the take-off performance calculation. The result of that incorrect take-off weight was to produce a thrust setting and take-off reference speeds that were lower than those required for the actual aircraft weight. The aircraft subsequently landed at Melbourne with no reported injuries. The tail strike resulted in substantial damage to the tail of the aircraft and damaged some airport lighting and the instrument landing system.
As a result of the accident, the aircraft operator has advised the Australian Transport Safety Bureau that it is reviewing a number of procedures including human factors involved in take-off performance data entry.
The investigation is continuing.