This study investigated Korean heritage speakers' Korean language proficiency (i.e. morphosyntax, collocation, and lexicon) in relation to their attitudes, motivation, and age at immigration (AI). Seventy-six adult Korean-Australians with Korean literacy participated. Overall, participants showed high levels of Korean proficiency in all three linguistic areas (over 80% of accuracy), and exhibited features of successful second/foreign language learners: a positive attitude toward learning Korean, strong interest in foreign languages, high motivation and self-efficacy, and low speaking anxiety. Their relatively high level of Korean proficiency can be explained by the crucial role of the family in maintaining the heritage language, supported by successive Australian governments' positive attitudes toward multiculturalism and active support from the local Korean community. Late-childhood bilinguals (AI: 9-12) were found to be the "ideal 1.5 generation," having the most positive attitudes and strongest motivations among the groups, while maintaining relatively high levels of Korean proficiency. This may be a result of frequent use of Korean at home and their self-perceived high proficiency in both Korean and English. Korean speaking anxiety was found to be the best predictor of Korean proficiency, implying room for instructional support. Providing explicit form-focused instruction and using Korean pop culture materials were discussed as pedagogical implications.