Three new oleanene-type triterpenoids, dillenic acids A [1], B [2], and C [3], and two known compounds, 3-oxoolean-1,12-dien-30-oic acid [4] (a new natural product) and the lupene derivative betulinaldehyde, have been isolated from the Papua New Guinean medicinal plant Dillenia papuana. The structures of the new compounds were elucidated as 2 alpha-hydroxy-3-oxoolean-12-en-30-oic acid, 2-oxo-3 beta-hydroxyolean-12-en-30-oic acid and 1 alpha-hydroxy-3-oxoolean-12-en-30-oic acid. The 1H- and 13C-nmr data of all new compounds were assigned unambiguously using a variety of 2D nmr experiments including 1H-1H-COSY, HMQC, HMBC, and NOESY experiments. Of these compounds, 1-4 showed antibacterial activities against Bacillus subtilis, Escherichia coli, and Micrococcus luteus.