The Warriors appeared on track for victory when 16-8 ahead with 20 minutes remaining but the Dolphins have been comeback specialists in 2026 and the electric Farnworth danced and weaved his way through to score.
A 90m movement, started and finished by Dolphins fullback Hamiso Tabuai-Fidow via a scintillating interchange of passing with Farnworth, put the Dolphins ahead 20-16.
Then Dolphins centre Jack Bostock iced the win with a ball steal to score and secure the Dolphins a home preliminary final at Suncorp Stadium in a fortnight.
A sold-out crowd of 26,131 at Mt Smart Stadium was pulsating and heaving throughout another classic battle between two sides that are fast forging a fierce rivalry.
They were silenced in the final 20 minutes by a Dolphins side that refused to be beaten.
"It is big. It is massive for everyone involved at the club," Dolphins coach Kristian Woolf said of the looming preliminary final.
"The club has done such a good job over the last four years in every department but the blokes that deserve the credit are the players with the way they played tonight and the bravery they showed under pressure.
"They are in this position for a reason and that's because they want to work hard for each other."
The Warriors will now host Newcastle in a semi-final next week, likely without strike centre Leka Halasima and returned prop Jackson Ford who injured an ankle in his first game since rupturing a pec 12 weeks ago.
The Warriors suffered a major blow with Halasima ruled out in the warmup after clutching at his right hamstring, with Adam Pompey slotting into the starting side.
Warriors coach Andrew Webster was asked about his emotions after the team let slip the chance to host a prelim final and he said he was "motivated".
"I don't think the we can sulk for long," he said.
"There is another path for us. It is not the direct path. We have got to move on quick. We have got to take lessons and get better, but we are in control of our own destiny."
This showdown was brutal, physical, fast and furious.
Warriors pair Pompey and Kurt Capewell, along with Dolphins winger Selwyn Cobbo, were all sent to the sin-bin in the first half.
Dolphins prop Thomas Flegler was concussed and ruled out of the match in the 10th minute after being steamrolled by opposite front-rower Mitch Barnett.
Pompey and Dolphins trio Isaiya Katoa, Connelly Lemuelu and Kodi Nikorima were all put on report as the clash boiled.
The match exploded into a melee in the 34th minute after Dolphins half Katoa flipped Ford accidentally in a dangerous tackle.
Capewell and Cobbo were both sin-binned for rushing in to the fracas and Katoa was put on report.
Every penalty or ruck infringement in the Warriors' favour was met with an almighty roar by the crowd.
The Warriors faithful were delirious when fullback Taine Tuaupiki danced past four Dolphins to open the scoring with a try under the sticks.
At 10-2 the Warriors were in control.
Farnworth went into the match with nine tries in seven matches against the Warriors but it was the Englishman's masterful tip-on to Cobbo that got the visitors their first try.
Pompey was sin-binned for a shoulder charge on Dolphins warhorse Ray Stone.
Tuaupiki kept landing penalty goals but the Dolphins scored the tries that sealed the deal.
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