The National's commercial indulgence in tackling the shows of Rodgers and Hammerstein is now, perhaps inevitably, obeying the law of diminishing returns. Nicholas Hytner's revelatory 1992 staging of Carousel was undoubtedly a tough act to follow; but Trevor Nunn managed it with a safe but solid Oklahoma! in 1998 that, like Carousel before it, subsequently transferred to the West End and is now Broadway-bound, too. South Pacific, however, is unlikely to travel any further than the South Bank.

While the film version by which this show is perhaps best known these days was famously shot in oversaturated technicolor, Trevor Nunn's dutiful production drains its drama to a dull, documentary monochrome. The earnestness of the approach is signalled the moment the show begins, with black and white documentary footage of wartime manoeuvres projected against a circular scrim that is wrapped around the front of the stage. As it parts, the opening number 'Bloody Mary' is being sung as a marching song as part of a military drill. You know it's going to be a long evening.

It is clearly Mr Nunn's intention to toughen the show up a bit. The gloppy romance at its centre, between Emile de Becque (the ever-reliable Philip Quast), an exiled planter from France who has found himself on the Polynesian island, and Nellie Forbush (appealing American newcomer Lauren Kennedy), a naïve navy nurse from Little Rock who is stationed there, is only part of story of South Pacific, of course. But it's the one that provides the show's richest, most resonant melodies, for which this show is justly renowned and beloved. Here are some of Richard Rodgers and Oscar Hammerstein's most yearningly beautiful ballads, 'Some Enchanted Evening' and 'This Nearly Was Mine', as well as a couple of their perkiest character numbers for the perennially perky Nellie, 'A Cockeyed Optimist' and 'I'm Gonna Wash that Man Right Outa My Hair'.

But Nunn seems more compelled by a military subplot which sees de Becque and Lt Joe Cable - a man with a romantic subplot of his own as he falls in love with the Polynesian daughter of the island fixer, Bloody Mary - despatched behind enemy lines.

Hammerstein and original director Joshua Logan's book, however, doesn't support this change of emphasis. While this is undoubtedly a show of darkness as well as delight, too much has here been cast into the shade. Meanwhile, the vaudeville-based second act concert number, 'Honey Bun', in which Nellie and Luther Billis entertain the troupes, finds the show seeking yet another change of pace.

These are partly flaws in the basic structure. But by playing the darker strain more heavily than the more prevalent lightness, it is fatally unbalanced. As, indeed, are the sound balances, which are amongst the worst I have ever experienced at a major musical. They mean that even if you want to simply treat the evening as an opportunity to hear a classic score again, it defeats you.

- Mark Shenton