A generous government allowance paid to some of Hong Kong's top-ranking civil servants without requiring them to account for the money has come under scrutiny as the cost to taxpayers looks set to exceed HK$1 billion annually by next year. The Non-Accountable Cash Allowance (NCA) is an expenditure that attracts little attention when the government opens its books every year, but with the ballooning total on track to be more than HK$1.1 billion in less than a year, questions have been asked as to whether it is sustainable. The NCA was introduced by the government in June 2000 to replace previous housing benefit allowances because it provided "flexibility for staff". "The NCA is fully non-accountable and is not tied to any housing expenses," a Civil Service Bureau spokeswoman told the South China Morning Post . Read more: Bill for standard working hours in Hong Kong could hit HK$10 billion, says committee report Read more: No one happy as Hong Kong minimum wage raised to HK$32.5 today But Andrew Shuen Pak-man, interim executive director of the Lion Rock Institute, a public policy think tank, said: "It is accounting trickery to make people think government size isn't growing and public servant pay is under control. But this is growing and nothing is sustainable at this growth rate." The allowance will cost the government an estimated HK$839 million in 2015-16, according to the 2015 budget, up 33 per cent on the HK$629 million spent in the previous year. Every year since 2006 the allowance has grown by more than a third - at that rate the figure will exceed HK$1.1 billion in the next financial year. In 2015, 4,000 of Hong Kong's more than 170,000 civil servants are expected to receive the allowance which averages out at about HK$17,500 each month, or more than the average Hong Kong salary. It is paid in addition to the civil servants' regular salary. Only 85 civil servants qualified for the allowance in 2005. According to the Civil Service Bureau spokeswoman, government employees who earn more than HK$60,000 a month automatically qualify for the allowance, while those earning less than HK$60,000 receive it based on a quota system and years of continuous service. The spokeswoman said civil servants could receive the allowance for a maximum of 10 years. "Due to the increasing number of officers attaining eligibility for the scheme through salary progression, promotion and new appointment, the number of recipients for the NCA and its expenditure have increased over the years," she explained. "The rates of the non-accountable cash allowance were drawn up by reference to the rates of other civil service benefit schemes for home purchase." But Shuen said while he was sympathetic to the government's need to attract high-quality candidates with large salaries, more transparency was necessary regarding such allowances. Former civil service minister Joseph Wong Wing-ping countered that the size of an allowance alone did not warrant it being reined in or re-evaluated. "You can look at this from an egalitarian viewpoint but that's not how the government runs the civil service," he said. Wong pointed out that the entire civil service pay package was approved by the legislature, including allowances, meaning the allowance was growing within its approved mechanism.