The most impressive new building in Melbourne is the £290m Southern Cross railway station. Its architects are Grimshaw, a British practice based in London. And the most impressive new building in Manchester is the 16-storey, £160m Civil Justice Centre. Its architects are Denton Corker Marshall (DCM), an Australian practice based in Melbourne.

Both Manchester and Melbourne are busy reinventing themselves. At heart, both are great Victorian cities that, following economic slumps, have been rapidly modernising. They are centres of sport and culture, their populations comprise a rainbow of backgrounds, and they boast impressive tram networks (trams make city centres special).

Yet both cities have been what you might call just a little bit provincial in their commissioning of new architecture over the past 20 years. Now, though, they are clearly benefiting from importing design talent from one side of the world to the other. Here are two cities that might well learn from one another, and that are finally embracing - if these two buildings are anything to go by - international architectural talent.

Southern Cross station is a delightful surprise. It acts as a junction box not just between Melbourne's long-distance and commuter trains, its buses, taxis and trams, but also between two quarters of the city until now separated by a freeway and the kind of urban nothingness that blights so many cities. These two areas are Melbourne's largely high-rise central business district, and its docklands, home to the Telstra sports stadium. The station sits between the two and holds their hands. As well as trains, it provides a pedestrian link between offices, the sports stadium and the redeveloping docklands.

Because it's such an important link, the station has been designed to stand out - but not in the manner of all too many visually incontinent "iconic" buildings. Its dune-like steel and aluminium roof, covering an entire city block, is its main attraction; it's certainly eye-catching, whether seen from the 14 platforms it shelters, where it looks like a lightweight blanket tossed into the air and now billowing back to earth; or from above, through the windows of Melbourne's office towers.

The shape of the roof is, though, a happily functional form, and its beauty, says Grimshaw's Mark Middleton, purely "accidental". I don't quite believe him, but the roof has certainly been thoughtfully designed to expel hot air and diesel fumes (the lines have yet to be electrified).

To assist the roof in its role as one great exhaust funnel, the glass walls beneath the roof do not meet the pavement, ushering in gentle breezes. A gap right around the largely transparent building ensures that, unlike the office blocks that overlook it, the station is, by and large, naturally ventilated.

In essence, the station is a giant parasol - and occasional umbrella, of course - showing how it's possible to build for a hot climate, and on a large scale, using natural ventilation. The benefit is not just a cool, smoke-free building, but one that enjoys a transparency designed to welcome passengers in, while looking after them gently inside. Railway staff occupy brightly coloured elevated pods set beneath the roof, leaving the entire ground floor free for passengers.

All this has been achieved over the past five years without suspending train services. Southern Cross is not yet a particularly busy station, but it will be the terminus for long-promised high-speed trains to Sydney, which might encourage Australian drivers out of their cars, together with a new service to Melbourne International Airport.

The station is not just important for Melbourne; it matters very much to Nicholas Grimshaw. This autumn, Eurostar trains will begin to streak out of St Pancras, making the Grimshaw-designed Waterloo International Terminal, a building that redefined the modern railway station, all but redundant. In a new era of high-speed trains, any self-respecting, internationally minded practice should have at least one world-class railway station in its portfolio.