The former Xena: Warrior Princess star was among a group of Greenpeace campaigners who climbed aboard the vessel in Taranaki in February (12) and chained themselves to the ship's tower to protest against oil drilling in the Arctic.
They spent four days onboard before they were removed by police and subsequently arrested.
Lawless and eight fellow activists appeared in court in New Zealand on Thursday and pleaded guilty to trespassing. Prosecutors had reduced a charge of burglary.
She faces three years in jail when she is sentenced in September (12).