The cost of renting office space in the skyscrapers of Sydney and Melbourne is rising faster than in any other major global city, as a lack of space and the disruption of a new rail line push up charges.Knight Frank, the estate agent, said that the cost of renting space in the upper floors of skyscrapers in Melbourne had risen by 11 per cent to £40.98 per square foot per year in the six months to the end of last year, while those in Sydney had risen 10.1 per cent to £78.39 per sq ft.In Sydney, right, the construction of a new Metro line has caused a huge loss of office space. The New South Wales government made more than 150 compulsory acquisitions of mostly commercial buildings to make way for the 66km line, which will have 31 new stations.Melbourne had the strongest take-up of office space of any city in Australia last year, while there is a limited amount of new space set to come on to the market this year. It is the second fastest-growing economy in Australia and office take-up is being backed by a surge in business confidence among the city’s tech and media companies.Hong Kong continues to be the most expensive city in Knight Frank’s 21-city skyscraper index, costing £243.69 per square foot per year, having risen 0.6 per cent in the six months to the end of 2016. Rents for skyscrapers in London were the fifth most expensive at £84.19 per square foot, but showed no growth.New York remained the second most expensive city at £128 while San Francisco was fourth highest at £90.98.