A drunken grandmother forced a New Zealand-bound Qantas flight back to Melbourne after she punched a passenger in the face, a court heard today.

Frances Macaskill, 58, from Perth, pleaded guilty in the Melbourne Magistrates Court to one charge of offensive and disorderly behaviour on board an aircraft and one charge of assault over the air rage incident.

Commonwealth prosecutor Lauren Gurry told the court Macaskill had been in seat 8F on board Qantas flight QF37 bound for Wellington at 9.20am on Saturday.

Ms Gurry said the air crew had observed Macaskill drinking alcohol she had bought duty free at Melbourne International Airport before boarding the plane.

"As the accused continued to yell profanities at other passengers and air crew, she then began to punch the seats in front of her in a violent manner," the prosecutor said.

Macaskill was told to stop but she ignored the cabin crew.

She then stood up and punched another passenger seated in 7E on the right side of the face, causing a six centimetre cut and heavy bleeding.

Macaskill was restrained by the cabin crew with the help of the victim and placed in flexi cuffs before being moved to an empty seat at the rear of the aircraft.

Ms Gurry said Macaskill continued to yell profanities and repeatedly headbutted the seat in front of her.

The crew was forced to attach three child seatbelts together and strap Macaskill across the shoulders to the seat to stop her from injuring herself.

At about 10am, the captain decided to return to Melbourne because of Macaskill's behaviour.

Federal police arrested Macaskill and noticed she was swearing loudly, slurring her words and was suffering the effects of alcohol.

The cost to Qantas to turn the flight around was A$19,760 (NZ$25,360).

Defence lawyer Nadia Moralis said Macaskill had no memory of what had happened and was shocked and appalled by what she had done.

Ms Moralis said Macaskill now recognised she had an underlying problem with alcohol.

She faces a maximum penalty of two years jail and A$5500 fine (NZ7060).

Macaskill was released on bail to be sentenced tomorrow.

- The Age