OFFICE rents in leading world cities are at their lowest for more than a decade but there are no bargains to be had in Hong Kong where rents have continued to rise, a report by property consultant Richard Ellis has found. Hong Kong was one of the few cities surveyed where rents increased in the past six months, mainly due to the tightening of available office space. It is the third most expensive city to rent office property in. Tokyo remained the most expensive city in the world where a square foot of prime office space costs about $130 per square foot per month. London, was the second most costly city at $60 per sq ft per month, and both it and Tokyo suffered a slight decline during the six months under review. According to the report, prime central business district rents in Hong Kong rose by 9.5 per cent to $51 per square foot per month in the six months to June this year. Richard Ellis director Dominic Leung said it was likely that Hong Kong could eventually become the most expensive city in the world. Tenants can expect to see rents rise in the prime business areas of Hong Kong by 20 to 25 per cent throughout the whole year as Grade A office vacancies decline to about five per cent, the report says. There will be no medium-term relief - short of a sudden drop in demand from tenants - as office vacancies are expected to fall as low as two per cent during 1995 or 1996 with little new space coming on line. Mr Leung said there were few opportunities for new office projects in the central business district and developers were looking at the fringe areas to buy land and construct buildings of a more substantial size. ''We are already seeing some major multi-national companies decentralising to the fringe areas where the office accommodation is still of a high standard but the costs are lower,'' he said. ''Other companies are moving their more labour-intensive divisions to parts of southern China or even Singapore but are maintaining their key operations in Hong Kong because of its proximity to China.'' The report says cheap space is available in several cities, which also offer lifestyle attractions, including Auckland, Los Angeles, Sydney and Toronto. The report says despite general availability of space in Bangkok, prime space on Wireless Road consistently achieved 600 baht per sq metre (HK$200 per sq m) and tenant demand is still high. Taipei - which is now more expensive than New York - suffered a slight decline in rental rates during the six months but still has the seventh highest office occupation costs in the world. European cities Paris, Frankfurt and Berlin followed closely behind Hong Kong for the cost of renting office space.