The West Australian resident had been flying home from a holiday in Chile on October 29 and was changing flights at Auckland Airport for the final leg of his trip.
But downing a bottle of vodka in the airport toilets derailed those plans.
Gunasekera was charged with indecent assault and threatening to kill, after a flashpoint which happened while the plane was still on the ground.
Mr Munro said his client could not remember anything of the incident.
"He's feeling really really embarrassed by what he did," he said.
Mr Munro said the man was extremely remorseful and would likely offer emotional harm reparation to the victim.
Gunasekera is alleged to have touched the breast of the woman sitting in the seat in front of him but Mr Munro said he believed there was no sexual intent in the act.
It is understood the woman stood up to confront him and the defendant said he thought she was someone else.
Since the incident, he had accepted he was an alcoholic and attended a residential rehabilitation programme at his own cost, which lasted a few weeks.
Now Gunasekera was taking medicine which made him violently ill if he consumed any alcohol, Mr Munro said.
The lawyer said his client was now feeling much healthier and had not drunk since.
The Qantas flight had been taxiing to the runway when the alleged incident happened.
It continued to its destination without incident after the defendant had been evicted from the flight and taken into police custody.
He will be back in court in April and Judge Johns excused his appearance, unless there is a resolution reached which would see him sentenced on that date.
