BMW’s Z4 has grown up, in more ways than one. So much so that perhaps it should be called the Z5, because the E89 represents even more of a quantum leap over the first Z4 than the latter did over the original Z3, which was introduced in Australia around the same time as its arch-rival, the Mercedes-Benz SLK, in early 1997.

It took more than two years for the Z3 Coupe to join the range, with the affectionately-named ‘munster mobile’ available here only as an M version.

The Z4 Coupe that replaced it in 2006 brought 3.0 and 3.2-litre M power, and while the latter continues to be available until sold out, the all-new Z4 Roadster replaces both the previous Z4 convertible and coupe.

Proving not even BMW can build a folding hard-top convertible that’s anywhere near as light as a soft-top, the new Z4 is almost a staggering 200kg heavier at base 2.5-litre level and similarly heavier in 3.0-litre guise, while the new Z4 sDrive35i flagship tops the range in twin-clutch M-DCT form at a very un-roadster-like 1600kg.

Throw in a slightly less powerful 2.5-litre entry-level engine and the base Z4 is a tenth of a second slower to 100km/h than its predecessor as well as significantly less efficient – although the six-speed auto version improves marginally.

It’s the same story with the less powerful Z4 sDrive 30i, which is a tenth tardier to 100km/h than the car it replaces with either transmission, as well as half a litre thirstier as a manual.

The auto has the same official fuel consumption figure as its forebear, which is quite a feat given its extra bulk, but hardly something to crow about for a car-maker whose mantra has become EfficientDynamics.

In lieu of replacements for the Z4 M Roadster or Coupe, which are yet to appear, the new Z4’s headline act is the additional sDrive35i variant, powered by the 225kW/400Nm twin-turbo direct-injection six from the 335i.

Despite also being almost 200kg porkier than the M Roadster, the Z4 35i is virtually as quick and more than two litres more efficient per 100km.

Mind you, as an auto, it’s also priced within $10,000 of both previous-generation Z4 M predecessors at more than $120,000, so expect big price rises for any future Z4 M Roadster too.

The rest of the Z4 range moves up in weight and price too. At base 2.5-litre level it is $6700 more expensive, while the 3.0-litre is a whole $8000 pricier than the convertible it replaces – and precisely $10,000 dearer than the Z4 3.0 Coupe, which was last sold at $88,100. And that’s before you factor in a plethora of expensive options.

Of course, all this is the bad news. Commensurate with its weight and price increase is the added safety, security and refinement of a fully integrated, power-operated folding aluminium roof.

It takes a relatively lengthy 20 seconds to open (during which time one must hold down the awkwardly positioned roof button) and cannot be operated on the move like some convertible roof systems, but it does it in virtual silence.

Thanks to a massively larger rear window and longer side glass, which now incorporates two small rear windows that can be independently opened, all-round vision is vastly improved. Impressively for a convertible, the rear three-quarter blind spot is no worse than in many coupes.

Styled inside and out by female BMW designers, the new Z4 not only moves up a size in weight – but also size and therefore interior comfort. The two-seater Z4 cabin is still cosy and cosseting, but there’s noticeably more head and shoulder room – even if the driver’s seat feels too high and its seatback bolstering is a tad too narrow for the, let’s say, wide backed among us.

The extra body length also increases total boot space, to 310 litres, but with the sizeable hard-top stowed it shrinks to just 180 litres, which is barely enough to store a couple of backpacks. With the roof down, wind buffeting in the cabin is acceptable up to 120km/h even without the dealer-accessory wind buffer, and normal conversation is can be had up to similar speeds.

While it’s less feminine on the outside, the new Z4 has the best interior yet. Entirely functional and ergonomic, there’s a little more storage space via fold-out door pockets and rear bulkhead compartments, while the fitting circular design theme is broken only by the optional flip-up dash monitor, which brings a new level of iDrive-operated technology to the Z4 but is so large it looks out of place.

The latest Z4 disguises its extra size and mass well on the road and continues to offer a poised, intimate driving experience. On a crisp Victorian morning with the roof down and the exhaust note reverberating against roadside cuttings, it still delivers a truly invigorating roadster driving experience.

No manual or even 2.5-litre versions were available to test during the extensive 500km-plus launch drive along the Great Ocean Road and surrounding Otway ranges, so we can’t say if the base sDrive25i lives up to its zesty predecessor.

But in 3.0-litre automatic guise the Z4 has lost little of the performance or agility of the model it replaces. Gone is the twitchy, elastic feel of the outgoing Z4’s electric steering, which is replaced by a more linear (but still electric) system that is suitably responsive and communicative, but still not as alive in your hands as the tillers of other conventional hydraulically-assisted BMW models.

Yes, the 3.0 seems as quick as ever, despite its larger overall size and mass, but even with a virtually identical wheelbase it delivers superior stability and roadholding. The revised independent rear suspension is a far cry from the twitchy Z-axle set-up of the original Z3, but still tends to clunk over big road bumps and dips.

Attached to an all-new chassis that feels even more rigid than before (although the roof did rattle on the odd occasion in our test car), the firmly set up, short-travel suspension performs an impressive compromise in terms of ride and handling, and irons out nasty road obstacles better than before.

There’s also a palpable difference in ride quality between the three adjustable damping settings (which also have a worthwhile influence on steering assistance and automatic transmission response), but we can’t help thinking how much better the ride would be on conventional (non-runflat) tyres.

The top-shelf 335i is considerably quicker again but in our hands on even twistier and more undulating target used less than 1.0L/100km more than the 3.0. It too offers push-pull toggle shifters on the steering wheel as standard, but when mated to the M-DCT twin-clutch gearbox the twin-turbo inline six produces all the effortless drivability of an auto with all the pure, unadulterated fun of a manual.

That said, the two-seater Z4 35i doesn’t feel any quicker than the 60kg-lighter, four-seater 335i Coupe, despite official figures claiming otherwise. Of course, there’s a tangible performance increase over the 335i (Coupe-)Convertible, which is 150kg heavier, but this two-seater roadster doesn’t feel as firecracker-like as the 115kg-lighter 135i Coupe, which BMW says is a tenth or two slower to 100km/h.

Similarly, the 135i Convertible, which has the same 1600kg kerb weight as the Z4 35i auto, is said to be half a second slower with either transmission – but for some reason the flagship Z4 doesn’t feel that quick.

Don’t get us wrong: the 35i Z4 is a rapid roadster that generates startling speeds with deceptive ease. It’s so effortless that it doesn’t feel anywhere near as lively as we remember the raucous-sounding, M-engined Z4 Coupe.

Like we said, the Z4 has grown up and the extra space, comfort and refinement will be welcomed by some, if they don’t mind paying the similarly inflated prices – which nonetheless on average still undercut SLK prices.

Perhaps the M version will rewrite the MkII Z4 rulebook, but we like our roadsters raw and, as it stands, BMW has headed too far away from the lightweight roadster concept realised by Mazda’s MX-5 and the original Z3, in an effort to emulate its fiercest foe in the SLK roadster/coupe.