However, when she was stopped at another checkpoint in Kingston the next day, she told police she lived in Edendale, Southland, and was returning there from Queenstown.
Appearing before Judge John Brandts-Giesen in the Queenstown District Court on Monday, McCrostie admitted a charge of failing to comply with an order under the Covid-19 Public Health Response Act, after previously being warned and issued an infringement notice, by continuing to travel between Cromwell, Queenstown and Edendale between August 24 and August 25.
The charge carries a maximum penalty of six months' prison or a $4000 fine.
Counsel Bryony Shackell said although the defendant lived in Cromwell, she was at her partner's home in Edendale when Alert Level 4 came into force last month.
She had done the right thing by remaining in Edendale, but had later decided to travel to Cromwell for her son's sixth birthday.
Judge Brandts-Giesen told the defendant he was not impressed by the explanations she had given police at the checkpoints and hoped she showed ''more sense'' in the future and stayed where she was supposed to be.
''Birthday parties for a child are important, but so is the health of the nation and the need for people to respect a lockdown.
''The rules are the rules for very good reasons.''
McCrostie, who was stopped by police for speeding on her way to court on Monday and had her driver's licence suspended, as a result, was convicted on the lockdown breach charge and sentenced to 80 hours' community work.