The Opals hit an incredible 12-20 (60%) from beyond the arc for the game and their precise long-range shooting proved the difference down the stretch against a dogged Japanese side.
With the victory, Australia finishes the pool stage on top of Group B but will have to wait for other results to find out their opponent in the Quarter-Final.
A win in that game will not only see them advance to the Semi-Finals of the Women's Asia Cup but also qualify them automatically for the FIBA World Cup to be held next year.
Against Japan, Marianna Tolo was brought back into the starting line-up after missing their clash with the Philippines.
The Opals took a couple of minutes to figure out Japan's full-court press, a tactic obviously taken from Australia's earlier encounter with Korea and it was a back-and-forth affair through the first term.
Kelsey Griffin scored five early points but Japan maintained a one-point edge at quarter-time thanks to 6-11 shooting from inside the arc.
Japan's defence was also impressive, cutting off Australia's attempts at interior passes and they held the Opals scoreless for six minutes across the first and second terms.
Belinda Snell (eight points, five assists) broke the drought for the Opals, scoring five points on back-to-back possessions and Griffin (16 points, nine rebounds, four assists) then went on a personal run herself with seven straight points.
Japan had the answers though, collecting nine assists on 12 field goals through the first half which helped maintain the margin to just four at the main break.
Australia's ball movement came into the game in the third period and their touch from the three-point line began to take effect, with Katie Ebzery (11 points on 3-3 shooting from deep) camping in the corner for three before Snell backed her up with another triple.
It capped a 10-2 run for the Opals to start the second half as they opened up a double-digit lead for the first time in the game.
Steph Blicavs (nine points on 3-3 long range shooting) and Griffin then kept the pressure on from beyond the arc but Japan were not going away, scoring quick baskets at the other end to keep the deficit manageable heading into the last term.
The Opals three-point barrage continued with Ebzery (twice) and Lauren Mansfield (five points, seven assists) connecting from deep inside the first two minutes of the fourth quarter but Yuki Miyazawa responded, penetrating the Aussie defence and trimming the margin to six inside the final three minutes.
Tolo (17 points, eight rebounds) had an answer herself though, knocking down consecutive baskets before finding Blicavs in the corner.
Her shot was ruled a two-pointer and Japan quickly scored at the other end but Blicavs (12 points, five rebounds) had another dagger up her sleeve, making sure her foot was behind the line this time to, fittingly, clinch the contest with another three-pointer.
Check out all the stats here.
Playing only three minutes, Alanna Smith came out of the game in the first half after a heavy fall and did not return with the team sure to keep an eye on her status.
The good news for Smith and the Opals is that they have a rest day tomorrow, giving them a chance to prepare for their Quarter-Final opponent.
Stay tuned to Basketball Australia's social media channels for news on their opponent, game time and broadcast details.
The Australian Opals FIBA Women's Asia Cup Schedule (all times AEST):
July 27: Quarter-Finals
July 28: Semi-Finals
July 29: Bronze and Gold medal games