State Commercial Taxes Department, which keeps a record of tax collection, said the drop is not expected to continue in the future.
The state government expects the GST to stabilise from January 2018 onwards.
"Since the implementation of the GST from July 1 this year, almost 30 per cent of the registered tax payers-cum- traders in Goa have not paid their taxes," a senior official from the department said.
As per the data available from the department, from July 1 till date, the department has earned Rs 491.79 crore revenue through GST (including VAT on petroleum), which is 22 per cent less as compared to similar period- July-September 2016, wherein Rs 628.27 crore were deposited with the state government.
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"The total shortfall is of Rs 136.48 crore," the official said.
He said the department is facing monthly shortfall of over Rs 40 crore as compared to the VAT collection and it (shortfall) will continue for another three months.
The official said that there is no need to worry about the shortfall as there is a provision that in the first five years, the Centre will compensate for the revenue loses.