Krystal Lee Reynolds, then 26, pleaded guilty to wounding the then 20-year-old woman on the left arm and left finger on November 27, 2017, in Invermay.

Crown prosecutor Emily Judd said Reynolds had been driving along Mayne Street in her Holden Commodore and another woman when she saw her ex-partner having a cigarette with the woman.

The court heard that words were exchanged before Reynolds got out of the car and pushed the younger woman to the chest.

Ms Judd said the younger woman threw a punch which hit Reynolds on the left eye.

Reynolds, who had exited the vehicle with the knife in her hand, used it to cut the woman on the left arm and the left finger.

Reynolds told police she thought the knife only touched clothes rather than what had caused a two-centimetre cut to the bicep and two-centimetre cut to the ring finger.

The court heard that Reynolds was convicted in the Launceston Magistrates Court for assault and resisting police over related incidents from the same day.

Defence counsel Mark Doyle said Reynolds response exceeded what was reasonable and that self-defence was not available.

Justice Michael Brett asked Mr Doyle: "Why did she get out of the car and why did she have a knife?".

Mr Doyle said she had wanted to talk to her former partner "Cowboy" about his infidelity.

"There was no need at all for my client to exit the vehicle and an extraordinarily unwise move which was likely to cause a confrontation," he said.

He said that Reynolds was reckless in causing the injury rather than intentionally doing so.

He asked the judge for punishment via a suspended sentence and community service order or a Home Detention Order.

Justice Brett said he was not satisfied that there was any significant remorse on Reynolds behalf and bailed her for sentence on August 7 at 10am.