The match went right to the wire in Dublin with the passionate Ireland fans roaring their side home.
Hosts Ireland won a nail-biting Women's Rugby World Cup Pool C opener against battling Australia in Dublin.
Larissa Muldoon's converted first-half try, against a Mahalia Murphy score, had Ireland 7-5 ahead at the interval.
Shannon Parry's touchdown edged the Wallaroos in front but replacements Ciara Griffin and Sophie Spence crossed to put Ireland on the road to victory.
The 2014 semi-finalists survived a late scare after prop Hilisha Samoa's try was converted by Ashleigh Hewson.
Ireland face Japan in their second group match on Sunday at the same venue, the UCD Bowl in Dublin, while Australia must try to re-group against France.
Defending champions England opened up this year's tournament by running in 10 tries in a 56-5 victory over Spain, while the USA beat Italy 24-12 in the other Pool B match.
New Zealand and Canada are the favourites to progress from Pool A and both started with victories, the Black Ferns beating Wales 44-12 and Canada winning 98-0 against newcomers Hong Kong, while France thrashed 14-man Japan 72-14 in the other match in Pool C.
Ireland aiming for semi-final repeat
Bereft of regular captain Niamh Briggs because of an Achilles injury, Ireland were led into the tournament by Claire Molloy as they set out to at least match their achievement of three years ago, when they lost to England in the last four.
Ireland, the 2013 and 2015 Women's Six Nations champions, went in as strong favourites, even though opponents Australia are just one place below them in the world rankings, in sixth.
Their opponents had lost all five matches they had played since the 2014 competition, albeit against highly ranked sides in England, Canada and New Zealand.
Australia have been concentrating on the Sevens version of the game, winning gold at the first Olympic tournament in Rio in 2016.
Yet they dominated territory and possession in the opening 10 minutes, before Ireland started to pile on the pressure.
Scrum-half Muldoon found a gap in the Australian defence midway through the half as he picked up and darted over for the match's first try, Nora Stapleton converting.
Australia were matching the Irish for physicality and responded eight minutes later when Murphy went over at full pelt in the corner after full-back Samantha Treherne sent an accurate looping ball over to the winger following a quick throw from a lineout.
Treherne was unable to add the extra two points after the ball fell off the tee just as she was about to kick the conversion.
Australia captain Parry barged over from close range 15 minutes after the resumption to nudge her side three points ahead, Treherne fluffing the conversion well wide of the posts.
A powerful drive by Ireland's forwards helped Griffin barge over for a converted try and Ireland looked to be well in control when Spence was adjudged by the television match official to have grounded the ball in the corner with 10 minutes left.
Soon after, Samoa managed to twist and turn and dot the ball down at the other end despite the attention of four Irish defenders who were unable to halt the progress of the Australia prop.
"That was a tough match. We knew Australia would be a physical side and they really put it up to us," Ireland captain Molloy told ITV after the game.
"I am proud of the grit, resilience and resolve shown by the girls. We knew we would create scoring opportunities if we stuck at it."
Ireland: H Tyrell; E Considine, J Murphy, S Naoupu, A Miller; N Stapleton, L Muldoon; L Peat, C Moloney, A Egan, P Fitzpatrick, ML Reilly, A Baxter, C Molloy, H O'Brien.
Replacements: L Lyons, R O'Reilly, C O'Connor, S Spence, C Griffin, N Cronin, K Fitzhenry, M Coyne.
Australia: S Treherne; N Marsters, K Sauvao, S Williams, M Murphy; T Pomare, K Barker; L Patu, C Campbell, H Samoa; C Butler, M Boyle; M Gray, S Parry (capt), G Hamilton
Replacements: E Robinson, V Tupuola, H Ngaha, R Clough, A Hewett, F Hake, S Riordan, A Hewson.