No marine heatwaves occurred, but the Great Barrier Reef remained in poor condition.

Oceans absorb 93% of excess heat from climate change, and global ocean temperature continued to rise. A new record was set, continuing a steady rising trend.
Global sea level continued to rise steadily by 6.2 mm, marking an increase of 72 mm since 2000 and 95 mm since 1993.
Sea level around Australia has been rising faster than the global average. The fastest rate of rise was in the Tasman Sea, which has risen by more than 150 mm since 1992.
Australian ocean surface temperature was lower than the previous year. Mean temperature was 0.09°C below the 2000–2018 average, but 0.32°C above the 1961-1990 average.

No marine heatwaves occurred, but it was the third-warmest year on record in the Tasman Sea. Cool sea temperatures prevailed northwest of Australia from June onwards, corresponding to a strong positive Indian Ocean Dipole.
The Great Barrier Reef remains in poor condition. No marine heatwaves of mass bleaching occurred, but health was poor in the northern reef and deteriorated in the central and southern reef with continuing Crown-of-Thorns starfish outbreaks.

source: Australia’s Environment in 2019 Summary Report (link)