It's funny how a song can take on a life of its own. When John Schumann wrote I Was Only 19 in 1982, I doubt he could have predicted where the song would go and take him with it. Introducing it tonight, he mentioned that at the time the rest of Redgum didn't like the song and so it was only he and Hugh McDonald (also his sole musical companion tonight) from the band who played on it.

The show tonight is organised by veterans' charity Legacy and timed to coincide with ANZAC Day. As well as the former Redgum members, there is poet Rupert McCall MCing the night and sharing poems about war, and an auction of photos taken in Afghanistan by soldier Tom Scheid.

If this seems like a strange setting for a pair of old left-wing folkies to find themselves playing at, it's definitely not a random occurrence. Over the course of the night we get anecdotes of John and Hugh being asked by a retired general to write a song for the centenary of ANZAC about aboriginal soldiers, playing at the funeral of a soldier killed in duty, even travelling to Afghanistan to play for the troops.

Through all the great songs, the strange metamorphosis of John Schumann into a kind of spokesman for Australian soldiers was the thing that kept running through my head. The incongruity really came to a head when they played Long Run. That first verse seemed kinda out of place in this occasion - "Australia marched out of Vietnam, out on the streets against Uncle Sam. We won the fight, it was a long one."

Given that they have come to be seen as symbols of a kind of Australiana (their evocative songs of the outback The Last Frontier and Diamantina Drover both get rousing singalongs tonight), it's easy to forget that the name Redgum was a pun - these folk musicians and philosophy students were communists. The same "Reds" that Australia went to Vietnam to kill.

In the time John Schumann has spent hanging out with soldiers he has picked up a fair bit of military slang which he uses liberally, but even so it is jarring to hear the language he uses to recount the story of a heroic ammunition drop in the battle of Long Tan - "our boys" surrounded by "the enemy". How things have changed. In all the war history of the night, it is never mentioned why Australia was ever in Vietnam (or Afghanistan, or Iraq).

Also conspicuously absent are any of the songs about Australian subservience to the US that used to litter the Redgum set back in the day. Even I've Been To Bali Too, that sarcastic attack on Australians abroad, has its lyrics changed to be about randy schoolkids. At one point Schumann, without a hint of irony, recounts a story that starts with him in Bali.

Of course the politics aren't entirely missing - there was a bit of drama when an audience member didn't take well to Schumann making derogatory remarks about Campbell Newman. At first, from the back, he called out somewhat misogynistic comments about Annastacia Palaszczuk. When the Newman bashing didn't immediately cease he walked all the way up to the stage continually yelling at the performers, and in the end had to be physically dragged off by the evening's MC Rupert McCall and a couple of others. I'm sure it's not the first time Redgum have been heckled for their political beliefs, but back in the 70's not many people would have paid $50 for the privilege to do it.

These days a Redgum show is many things. As well as what I've already mentioned there was plenty of humour between songs, guitar heroics and great harmony singing from Hugh McDonald, a couple of songs from the album of Henry Lawson poems John and Hugh recorded in 2005, and a folky cover of Cold Chisel's Khe Sanh off a more recent album of Australian songs about war.

And then, finally, I Was Only 19; that extraordinary song that has come to overshadow all the rest of Redgum's career. John Schumann may have never fought in a war, but his representation of the soldier experience has come to be a song of immense importance for Australian troops stationed overseas. Seeing it sung by a room full of people, many of whom have lived out those horrific experiences detailed in the lyrics, was one of those spine-tingling moments that lifts the event from just another show to a truly memorable experience. One of the biggest cheers came in the song's introduction when John Schumann said, "One day hopefully we can work out a way to solve problems without shooting at each other.”

- Andy Paine