THE Government is in line for another cash windfall with land revenue likely to greatly exceed its original estimate of $14.1 billion. The booming property market in 1991 resulted in a record windfall of more than $20 billion in land income during the last financial year and the Government expects the trend to continue. ''With the market continuing to be good, I think even for the year 1992-93, the land revenue will certainly exceed estimates,'' the Secretary for Planning, Environment and Lands, Mr Tony Eason, said yesterday. ''On the land side, we are almost bound to be a bit conservative. I don't think it's prudent to predict on the basis of a booming market.'' Government accounts show that for the first six months of the financial year, more than $6.2 billion was collected, with $3.6 billion coming from sales at public auction and tenders. During that six-month period, about $2.1 billion was collected from modification of existing leases and another $354.7 million from private treaty grants. A record land auction price was realised on Wednesday with the World International (Holdings), Wharf (Holdings) and Hongkong Realty and Trust paying $3.53 billion for a single plot of land. Yield from the 280,510 square foot Diamond Hill site is expected to boost the healthy public funds. Mr Eason said the Government was still working on next year's land disposal programme but he expected the 1993-94 plan would be similar to that for the past two years. The Sino-British Land Commission agreed last March that up to 159 hectares of land be disposed of in the current financial year with about 28 hectares earmarked for commercial, residential and industrial development. Normally, the two sides of the Land Commission meets in late March to discuss and agree on the land disposal programme for the coming financial year. Despite the ongoing row between China and Britain over Hongkong's constitutional reforms, Mr Eason, who is also the British team leader of the Land Commission, said work at the land body would not be affected. Mr Eason was also confident that discussions on the Chek Lap Kok airport land requirements will continue. Noting the land body had held discussions on the Chek Lap Kok airport land requirements in the past year, Mr Eason said: ''There is no immediate problem on land grants, no time constraints.''