FIRMS in Hong Kong's Central Business District have been taking action to combat rent rises which are widely expected to continue for some time to come. Prime office space is tight, with the prospect of no significant new supply until 1996. Some agents doing what they do best - talking up the market - have predicted 100 per cent jumps over the next two years. Most analysts, who cannot afford to be seen making such wild forecasts, are sticking to a relatively conservative rise of around 20 per cent this year. But another school of thought exists. The lack of activity, particularly in the sales sector, which has characterised the market in recent months, is leading to private concern in the industry. Some think the market has reached its peak and a correction of around five to 10 per cent has been muted in some areas. Quietly, though, some property experts are worried Hong Kong's notoriously up and down market could go into a more severe dive. Firstly, they say the space situation in Central is not as critical as the agents would have you believe. The tightening of China's economy, and a possible failure by the United States to renew Most Favoured Nation status in China, would badly affect the territory. Foreign firms might stop arriving in droves, while those already here might slim down their operations or pull out altogether. The result will be a drop in demand and an increase in supply. Whatever the experts believe, company bosses in Central have been re-evaluating their operating costs and, in some cases, packing up. For the last year, firms have been moving out to decentralised locations in bigger numbers, or sitting down with landlords to restructure leases long before the existing ones are due to expire. Citibank Plaza is slowly edging towards full occupancy. Agents claim supply in Central is now virtually non-existent. Evergo's Entertainment Building in Queen's Road, which only has a modest 6,400 square feet floor space, is officially being leased. But, in reality, the firm is waiting for the right moment to pounce on a sector desperate for quality offices. That moment is likely to come early next year and rents are sure to be the highest in town. Central Development's redevelopment of Shell House and Hartlane House, just across the road, is now underway and adding to the shortage. Hong Kong Land, the owners of the traditionally most expensive offices at Exchange Square, is asking over $70 per sq ft for the limited space still available. Evergo's Lau brothers are slightly lower at $67 in the Entertainment Building. The average rates in Central are more than $50 again. The strong demand from foreign firms moving into the territory has helped turn the market in favour of the landlord. In mid-1989, rentals averaged $67 before the bottom fell out of the market and rents hit $44 last year. Tenants who cashed in on the slump and moved to new homes with better terms have been, and still are, looking at another move. A number of firms have moved ''back-room'' operations out of Central to cheaper locations like Wan Chai, Causeway Bay and Quarry Bay. But that has left space in these areas in shorter supply, too, pushing rentals up. The new Times Square, where the second tower is now being leased, has filled up far more quickly than expected and rents are up to nearly $40 per sq ft. In Wan Chai and Causeway Bay, according to Jones Lang Wootton statistics, just under two million sq ft of accommodation was completed last year. A further 1.2 million sq ft is scheduled for completion this year, with limited supply of 162,000 sq ft next year and in 1995. Despite the expected shortage in Central, Lyall Alexander Webber, a director at Vigers' commercial department, said he was confident the situation was not critical. Despite the fact only 326,000 sq ft of new space is expected to come on stream in Central between now and 1995, companies that are decentralising will ease the pressure.