IN SEVEN years, the Mercedes-Benz M-class has established an important role in the company's Australian model mix.

So important, that in 2001 at the apogee of its flight on sales charts here it was the marque's second strongest-selling model, behind the C-class but way in front of any other Benz.

At that time, the M-class was a formidable presence on the prestige four-wheel drive market, not far behind Land Rover's segment-leading Discovery and outselling BMW's recently launched X5.

Fortunes have changed since then. The BMW moved to the top in 2003 and never went away, while the Land Rover and the Benz have been on a steady decline.

For the latter two, that could be about to change.

The all-new Discovery is now arriving in steady numbers, and Mercedes-Benz has just announced the all-new M-class, which steps up to the mark with more of everything except weight.

It is a hulking brute of a thing, nevertheless.

It's bigger than the X5, virtually as big as the new Discovery - and way longer in wheelbase - yet tipping the scales on average 50kg below equivalent previous M-class models.

Prices have jumped, too, by 8.1 per cent with the ML350 and 11 per cent with the ML500. The turbo-diesel model in the M-class range will be also dearer, although Mercedes argues there is no easy calculation given the differences between the forthcoming ML320 CDI and its predecessor.

The Mercedes 4WD has undergone a big transformation.

It suggests bulk, strength and space, backed up by improved off- and on-road abilities and a more comfortable place in the line-up as a true Benz in terms of apparent quality.

The M-class now gets a full unitary body rather than a ladder-frame chassis, adding significantly to structural strength yet contributing to the weight loss.

It also gets two all-new, all-alloy V6 engines, petrol or diesel-powered (the latter follows in a couple of months), as well as a slightly refreshed 5.0-litre V8 in the ML500 that produces 10 more kiloWatts of power.

The suspensions are new, too, with a refined double-wishbone system up front and an all-new multi-link independent arrangement at the rear replacing the previous model's double-wishbone set-up.

Mercedes also claims the M-class is the safest 4WD on the road. It adopts things like the Pre-Safe system (which battens everything down in the event of an imminent collision), the body has further improved crash energy absorption and there is a host of electronic systems to help keep the vehicle safe and stable if the driver commits an indiscretion.

There are also Neck Pro anti-whiplash front headrests, two-stage front airbags, front and rear side airbags as well as full-length curtain airbags.

Off-road credentials are improved, and will be further enhanced when the choice of an optional "Off Road Pro" package is offered with the arrival of the turbo-diesel.

This adds a two-speed transfer case vital for off-road scrabbling, a sump guard and lockable centre and rear differentials. It also gets an Off-Road Pro version of the Airmatic hydraulic suspension that, among other things, allows the driver to select any one of three jacked-up ride heights from 181mm to 291mm, improving ground clearance and increasing the wading depth for river crossings.

The standard Airmatic fitted in the ML500 allows the vehicle to be lifted 80mm above regular ride height.

To tackle light-duty off-road work, the M-class is pretty well equipped in standard form. On top of its full-time three-differential 4WD system that distributes power 50-50 between front and rear axles, it gets an adaptable traction control system (4ETS), hill descent control (DSR), Start-Off Assist (to take the anxiety out of getting under way on a steep incline), off-road ABS calibration and a switchable electronic regime that modifies the transmission shift points and accelerator response to allow more accurate off-road control of the vehicle.

The 3.5-litre V6 replaces the 3.7-litre 18-valve engine used in the previous ML350 and, as well as being all-alloy, adopts twin overhead camshafts per cylinder bank and goes from three to four valves per cylinder.

Despite the smaller capacity, the new V6 develops more power (200kW) than the outgoing 175kW engine.

The M-class’s reduced weight, improved aerodynamics (the Cd drops impressively from 0.40 to 0.34) and improved engine efficiency sees not just a lift in performance – it will reach 100km/h in a quick 8.4 seconds – but also a reduction of fuel consumption which, in the ML350, is now quoted at 11.5L/100km compared with 12.5L/100km for the 3.7-litre.

Upping the fuel capacity from 70 to 95 litres also vastly improves the cruising range.

There is another contributor to the improved economy and performance, as well.

The M-class, in all versions, now uses the Benz 7G-Tronic seven-speed automatic transmission that gives a seamless progression of power while allowing the engine to tick over at lower rpm on the highway.

It is controlled, BMW 7 Series-style, by a wand to the right of the steering column, and by rocker buttons behind the steering wheel spokes that allow the driver to shift up or down manually.

The centre console is free of any gear-shifting protuberances.

The M-class’s bigger dimensions translate to a bigger interior, too, with a claimed 15mm increase in the space between front and rear seats, and 35mm more knee-room for the rear passengers, who also get 32mm added elbow-room.

Specifying optional removable rear cushions for the 60/40 split-fold rear seat can stretch the load area length to 2.1 metres and provide a maximum load capacity of 2050 litres.

Under the rear floor resides a space-saver spare tyre, which Mercedes almost apologetically says is at least fully inflated.

The new M-class is aimed clearly at the BMW X5 and Mercedes hopes it will drag some customers back into the fold who had moved over to the Munich brand on experiencing its obvious dynamic superiority.

The M-class plant at Tuscaloosa in the United States has been given a major $US600 million upgrade for the new vehicle, coping with both the extra volume and improved quality that has been demanded by the market.

Tuscaloosa was originally designed to produce around 60,000 M-class models a year but the model’s initial popularity was such that it was soon churning out 100,000 per annum.

In Australia, M-class so far has represented something like two percent of the worldwide market, with 13,000 Australian purchases since its 1998 launch. More than 650,000 have been sold worldwide.

Hopes are that the new vehicle will climb past the 2749 sales in achieved in 2001, topping 3000 and approaching archrival X5 next year.

Ironically, 3000 annual sales are also what Land Rover is targeting for its new Discovery.