As the cast and crew are told to move well back and block their ears, Beth Allen and Ido Drent look slightly worried.

The Shortland Street stars watch as men in fireproof vests make final preparations ahead of a car explosion - the climax of the TV2 show's 90-minute winter special.

Allen (Brook Freeman) and Drent (Daniel Potts) believe their friend, Lee Donoghue, who plays wayward medical student Hunter McKay, might be trapped in the car as it blows up, sending flames shooting about 40m into the sky.

"Even after all the warnings they got there is still a real shock on their faces," said producer Steven Zanoski. "You can see them jump, it's not acting."

But Allen was putting on a brave face, adding: "I was thinking it was going to be a lot more terrifying than it actually was."

The feature-length special, which screens tomorrow night, is the second in the soap's 19-year history. It includes a wedding and a birth, as well as the impressive pyrotechnics.

Without giving too much away, Zanoski said that Donoghue would soon be leaving Shortland Street.

Summarising the plot, Zanoski said: "He has been taking prescription drugs that have been spinning him out. He is trying to go cold turkey then finds out his father is sleeping with his girlfriend. He sees something that he shouldn't at the wedding and drives off. Feeling betrayed, he spins off the road and ends up in his own burning car."

The movie-length episode allowed producers to pull off a few big stunts.

A 12-hour day shooting in a field west of Auckland will make four to five minutes of television. "We get a lot of bang for our buck."