Immigration staff were owed up to 195,000 hours in overtime by the end of June - about eight working days for each eligible employee if they took an equal share.
And that is despite the fact almost 160,000 hours had been paid off.
Staff are worried about how they will be compensated for the outstanding hours.
Official figures showed 94,802 hours were owed at the end of June last year. The figure fell slightly to 89,777 by the end of September, but by the end of December had reached 92,293.
The amount had climbed to 149,926 by the end of March and alarm bells were ringing at the end of June.
Immigration Services Officers' Association chairman Wong Shiu-wing said: 'We are told the hours accumulated by the end of June are greater than 140,000. It is estimated to be 20 to 30 per cent higher.' The estimate of up to 195,000 hours is despite the fact that between April and June this year, 159,737 hours of compensation had been granted.
'This indicates the workload is increasingly heavy. Because of inadequate resources, the situation can only get worse,' Mr Wong said. The union is demanding the Government allocate more money to the department.