Sparked by the interesting observation of non-native acquisition of heritage languages despite early and continuous exposure, the study of heritage languages has endeavoured to explore the end results of heritage language acquisition while mostly neglecting the pathway that heritage speakers undergo before arriving in that state. This study investigated how the heritage language develops and is maintained over primary school years in heritage speakers of Korean who grow up in Australia. Linguistic abilities in Korean of 243 heritage speakers of Korean of primary school age in Australia, compared to their school-year-level-matched native speakers of Korean in the Republic of Korean (South Korea), have been examined in the three broad linguistic areas of the sound system, lexis and grammar with a battery of tasks. The results of the tasks indicated that the heritage speakers generally fell behind the native speakers in the linguistic abilities in Korean examined but their gap to the native speaker controls differed between the linguistic areas examined. Regarding the sound system, the heritage speakers did not show reliably lower perception of speech sounds in Korean than those of the native speakers, and their speech sound perception, which is supposed to have developed and been stabilised in early childhood, appeared to be retained well over primary school years. On the contrary, they exhibited a great shortfall in lexical knowledge in Korean compared to their native speaker peers, and this gap appeared to widen from the middle primary school year levels where the lexical knowledge of the native speakers expands explosively. In comparison, their linguistic abilities in grammar exhibited a varying degree of divergence to the native speaker norms by linguistic aspect. The results suggested that they acquired basic syntactic structures and semantic features that develop early in first language acquisition of Korean to a level comparable to that of their native speaker peers and their understanding of sentences made of such linguistic aspects was maintained well over the primary school period. In contrast, they exhibited a considerable delay in the acquisition of certain grammatical aspects that are mastered relatively late by the native speakers, and the heritage speakers’ acquisition of these aspects did not seem to progress greatly over primary school years. They also showed a substantial gap to the native speaker controls in understanding passive sentences and scrambled active sentences, and this gap is likely to have arisen from their greater processing difficulty. Mostly paralleling the linguistic abilities of adult heritage speakers attested in previous research, the results of this study underline that the linguistic abilities in Korean of heritage speakers of Korean in Australia diverge from their (age- or) school-year-level-appropriate native norms already in their primary school years. Although the linguistic aspects that are mastered early in first language acquisition such as phoneme distinction or basic syntactic structures seem to be acquired to a level comparable to their school-year-level-appropriate native norms and be retained well over the first half of their compulsory schooling, the linguistic abilities that should develop further through primary school years show signs of stagnation (if not attrition) and such signs are much more prominent at the middle primary school year levels. This implies that in Australia where EnglishKorean bilingual education programs are not readily accessible to heritage speakers of Korean, it will be extremely difficult for them to develop high proficiency in Korean which requires mastery of complex grammatical aspects and extensive vocabulary. The results of this study not only alarm the Korean ethnolinguistic community in Australia and other stakeholders about the level of Korean language abilities developed and maintained by the heritage speakers in primary school years, but also provide detailed information on in which linguistic aspects they may have greater delays in the development, by which degree they show such delays and when the delays are likely to intensify over the course of primary school years.