In Paraguayan Guaraní (PG), nasalisation processes affect material to both the left and right of a stressed nasal vowel. While some prior literature has claimed that bidirectional harmony is active in the language, others have noted that progressive nasalisation appears to be morpheme-specific and likely dependent on a different mechanism than regressive nasal harmony. Recent work shows that Spanish-origin lexical items participate in regressive nasal harmony, but the interactions of etymological origin and progressive nasalisation remain unclear. Drawing on a corpus of 26 sociolinguistic interviews as well as elicitation with native speakers of PG, I argue that the mechanisms underlying the two types of nasalisation in the language are in fact different. I propose that PG regressive nasalisation is best analyzed as productive nasal harmony, while progressive nasalisation represents a case of morpheme-specific allomorphy. Additionally, though the PG pattern of regressive nasal harmony has been extended to items of Spanish origin, this is not the case for progressive nasalisation. This corpus study provides invaluable insight into the specific factors that condition variation in nasalisation processes, contributing to a growing literature investigating variable application of harmony.