The man grabs the woman from behind by her shoulders, but she fends him off. The woman then chases the alleged attacked away and he gets into his car and quickly drives off.
Counties Manukau South area commander Inspector Joe Hunter confirmed police were investigating an assault and theft at the petrol station on Sunday.
The victim was putting petrol in her car at 8.40pm when she was attacked.
"A male has approached her asking for her number, when she refused his request, he has assaulted her and stolen her necklace.
"The male quickly left the scene driving a stolen vehicle which has since been recovered.
Hunter said the victim was "understandably shaken" and went to her mother's house to call police.
"The car used by the offender has since been recovered and forensic examination has taken place. Police have reviewed CCTV and are continuing to make inquiries."
Hunter said the man was described as being Māori or Pacific, in his late 20s, with facial hair and dark brown hair.
Anyone with information is asked to contact police, quoting file number 211019/3766.