The COVID-19 pandemic has created many challenges for adolescents across the world. The current study aimed to investigate the prevalence of depressive and anxiety symptoms and suicidal ideation in adolescents living in Melbourne, Australia, during the state of Victoria's second lockdown. The study also sought to identify pre-existing and current psychosocial stressors associated with adolescent's depressive and anxiety symptoms, and to identify the extent to which adolescents experiencing mental health difficulties sought professional help during the pandemic. A COVID-19 sub-study of the Mothers' and Young People's Study--an 18-year longitudinal cohort study--was conducted between July and September 2020, an online survey completed by 257 adolescents aged 14-17 years and their mothers, which asked about pandemic-related stressors, remote learning, family life, and mental health. Descriptive statistics and hierarchical multiple regression analysis were conducted. Mental health difficulties were common with 38% and 20% of adolescents reporting clinically significant depressive and anxiety symptoms, respectively, and 21% reporting frequent suicidal or self-harm ideation. Factors associated with depressive and anxiety symptoms included being female, exposure to current maternal depressive symptoms, lower levels of resilience, experiences of loneliness, stressful life events, and school- and family-related stressors. Two-thirds of adolescents who were experiencing clinically significant depressive or anxiety symptoms had not sought professional help. Our findings highlight the urgent mental health need among adolescents and the importance of reducing barriers to accessing support.