A study examining the composition and dynamics of the soil seedbank was conducted at two locations in central Queensland between December 2007 and May 2009. These two grassland communities were infested with parthenium weed (Parthenium hysterophorus L.), which had been present at both sites for at least 25 years. During the period of study, the seedbank varied between 5 962 and 16 206 seeds/m² at the Clermont site and between 6 795 and 24 862 seeds/m² at the Moolayember Creek site. Parthenium weed exhibited a very abundant and persistent seedbank, accounting for 80–87% of the seedbank at the Clermont site and 3–26% of the seedbank at the Moolayember Creek site. The species richness and species diversity of the seedbank, as well as the seed abundance of several native and introduced species, were higher at the Moolayember Creek site than at the Clermont site. The domination of the seedbanks by parthenium weed, especially at Clermont, suggests that the weed is having a substantial negative impact on seedbanks of native plant communities. The diversity of the seedbank at the Clermont site was found to be lower in comparison with that observed during an earlier study in 1995–1996, while the diversity at Moolayember Creek was found to have increased. Hence, the prolonged presence of parthenium weed may have substantially reduced the diversity of the seedbank at the Clermont site, while improved management practices may have increased diversity at the Moolayember Creek site.