THE Government faces the embarrassing threat of its budget surplus soaring still further, as it seeks approval for an extension to the Hong Kong Convention and Exhibition Centre. Secretary for Trade and Industry Brian Chau Tak-hay warned yesterday that the whole project might have to be scrapped if councillors, who will be asked to approve it again this Friday, continue to oppose the Government's proposed financial arrangements for the extension. But the immediate effect will be to increase the budget surplus - estimated to be between $15 billion and $20 billion - by $4.8 billion, unless the project is approved before the end of the current financial year. Financial Secretary Sir Hamish Macleod's financial estimates for this year's March 2 address have already been made on the assumption councillors would approve the project. Mr Chau said he would intensively lobby legislators ahead of the vote on the granting of $4.829 million to the Trade Development Council (TDC) to cover the cost of building the extension. If it is not passed on Friday, the Government will only have one other opportunity to seek approval before the end of the financial year on March 31. Legislators Martin Lee Chu-ming and Chim Pui-chung said yesterday they had yet to be convinced of the Government's case. The extension, which will more than double the amount of space for exhibitions and conventions, is expected to open in 1997.