New Zealand's longest-running soap opera celebrated 25 years on our screens this evening by bringing back some favourite characters - but it was an erupting Mt Ferndale that played the starring role.

Earthquakes were the first sign that something wasn't right in Ferndale, but guests at Chris Warner's 50th birthday were determined not to let the tremors spoil the party.

Dr Sleaze - as he was dubbed in the pilot episode - is dying of leukaemia. It was touch and go whether the character, played by Michael Galvin, would make it to his birthday.

He got there in the end but the night was marred by tragedy when Mt Ferndale erupted, spreading a deadly cloud of ash and poisonous gas over Auckland.

As the injured and choking victims flooded in to Shortland Street Hospital, Lucy gave birth to her baby in a car half-buried in volcanic ash. Although the baby was healthy, the birth went badly and Lucy was bleeding out - the episode ended without divulging her fate, leaving many fearing the worst.

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Several familiar faces were back for the feature-length episode.

Martin Henderson, who reprised his role as Stuart Neilson, told the Herald he felt "like a ghost" wandering the corridors of his old hospital, which propelled him to stardom in the 1990s.

Nick and Waverly made an appearance, Lionel Skeggins surprised fans from the 1990s (he was thought dead after he was swept off a rock by a rogue wave in 1999) and beloved receptionist Marj Brasch returned - passing away peacefully in her hospital bed at the end of the episode.

Reaction to tonight's show on social media was celebratory. Rosa Kome called the show "an emotional rollercoaster" while Debbie Munro said the "mind-blowing" Shorty St was the "best episode ever".

Die-hard fan Bronwyn Greer even posted on Facebook that she had cooked a celebratory dinner for the occasion, complete with "roast lamb, baked spuds, veg, gravy followed by chocolate self saucing pud".

And although there were some complaints about the realism of the volcanic eruption, GeoNet - which the show's creators approached for research prior to the episode - has given it an 8/10 for scientific accuracy.

A GeoNet blog post said after viewing the episode the team of volcano scientists had "mixed feelings around the quality of acting" but generally approved of the depiction.