But there were no fits of diva temperament on display at rehearsals on Thursday from the headliners: Australian baritone Peter Coleman-Wright, New Zealand tenor Simon O'Neill and Latvian soprano Inessa Galante, all with well-established careers in opera houses around the world.

The Bureau of Meteorology is forecasting a 60 per cent chance of showers on Sunday, including a possible thunderstorm in the afternoon or evening. But Latham said the event could go ahead in the rain.

"We have 3200 ponchos in the aisles, and we're encouraging people to bring wet-weather gear," he said.

Latham says this concert - the fourth Voices in the Forest - features "the best line-up of singers we've ever had."

Supported by Canberra soloists, the Woden Valley Youth Choir and an orchestra conducted by Roland Peelman, they will present a concert of opera, operetta and musical theatre favourites along with some lesser-known pieces.

With a baritone and a tenor among the soloists, the concert will include one of the most popular operatic duets: Au fond du temple saint from Bizet's Les pecheurs de perles.

O'Neill, who has worked often with Coleman-Wright, said: "Two of our great heroes are Jussi Bjorling and Robert Merrill, who recorded a famous recording of the duet."

They will also perform a duet from Verdi's Otello. Placido Domingo was famous for his performances as Othello. O'Neill studied under him and has played many of the roles formerly sung by the Spanish tenor, who has moved into baritone repertoire.

"He's left them all to me," O'Neill joked.

He said his young daughter had encouraged him to sing Katy Perry's Roar but that he would stick to more traditional fare, including Nessun Dorma from Puccini's Turandot.

Also new in Voices in the Forest are arias from Wagner's Die Walkure and Massenet's Thais, as well as extended excerpts from a single opera, Puccini's Madama Butterfly. These will include the Humming Chorus and the Flower Duet - but not Un bel di as originally planned.

According to Latham, Galante said she would prefer to sing the duet, which she will perform with Christina Wilson, and he could only agree. His main job, he said, was to ensure the performers were as happy and comfortable as possible.

"The singers will do very well when they feel very good."

And besides their instincts after years of performing were usually sound, he said.

Bob Winnel, the managing director of Village Building Company, which has invested a total of $250,000 in Voices in the Forest since it began, said: "I love classical music and Broadway music." The show was intended to have broad audience appeal, he said.

Voices in the Forest is on at the National Arboretum, Canberra, on Sunday, November 30 at 4.45pm. Gates open at 1.30pm. Website: voicesintheforest.com.au and tickets from canberraticketing.com.au.