"The government itself has gone public saying that it could not go the full distance," Jayampathi Wickramaratna, a constitutional lawyer who was involved in drafting the 19A, said.
The 19th amendment (19A) received 215 votes in favour with just one vote against it in the 225-member assembly.
"If you are talking of percentages the powers of the presidency have been reduced by about 60 to 65 per cent," Wickramaratna said.
Introduction of the 19A was a major election pledge of president Maithripala Sirisena during his election campaign. He defeated Mahinda Rajapaksa on a broader programme of democratic reform.
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The bill removed the president's power to dissolve parliament, until it has completed four and a half years of its five-year term.
Wickramaratna said the setting up of 11 commissions to depoliticise important areas of governance was a key feature of the 19A.
Since the adoption of the 1978 constitution there has been a big public outcry to abolish the system.