An increasing scarcity of water for irrigation and of arable land, decreasing agricultural land fertility, reducing forest cover, coastal erosion, and loss of bio-diversity were some of the environmental challenges the island faced.
A multiplicity of ministries with overlapping functions made it even more difficult to enforce already weak implementation of existing regulations.
A three-tied government structure - central, provincial and local - had slowed down and confused decision making, Gunawansa said.
"We have to ask whether coalition governments have contributed to inefficiency with too many cabinet ministers and subjects overlapping.
"So how do you have a proper policy framework and implement policies if there is so much overlap between government ministries?" he asked, referring to the plethora of ministries created by the ruling party to attract opposition members to strengthen government ranks.
Most of Sri Lanka's ruling party members of parliament hold a ministerial position of some sort.
Eradicating poverty was a key factor in protecting the environment, Gunawansa told a forum on environment management challenges in Sri Lanka and Singapore.
"Despite high economic growth there is a growing disparity in incomes and high poverty levels. The rich are getting richer and the poor, poorer," Gunawansa said.
"The challenge is to achieve sustainable economic growth with greater equity. We have peace and stability today but how do we ensure equitable growth where everybody benefits from economic development happening in the country?" he asked.
"Environmental development has to be in harmony with economic development," he said. "If not, you'll have beautiful green fields but a lot of poor people."
Economic growth, which had averaged around 4-5 percent during the island's 30-year ethnic conflict, hit eight percent after the war ended in 2009, but will slow down this year.
"The environmental law is too old and does not have enough teeth to deal with problems we have," Gunawansa said.
He recalled how in 1997 a new law was drafted by a committee of officials of which he was a member.
"Two months later the government changed and nothing happened. Our effort was wasted."
Sri Lanka has achieved most of the Millennium Development Goals set by the United Nations but not in ensuring environmental sustainability, Gunawansa said.
"Regulations and environmental laws are not properly implemented," he said.
"Setting goals is not enough. You need adequate guidelines and implementation. Also, sustainable development should benefit all.
He also warned against "hasty" infrastructure development which he said "often happens at the cost of environmental degradation."Gunawansa noted how in Sri Lanka often environmental impact assessments, known as EIAs, are not done with projects passed either owing to political clout or corruption, resulting in damage to the environment.
"So we must ensure good EIAs are done before any infrastructure project is undertaken."
The country has to deal with the problem by avoiding and mitigating risk and applying the law effectively.
This means preventing pollution at source by means of risk reduction, using the polluter pays principle which requires the cost of pollution to be borne by those who cause it.
Care of the environment should be the common task of both state and citizen.
"You must have 'top-down' and 'bottom-up' approaches, through better education, proper guidelines, and implementation from the top on the principle of sustainability," he said.
"The right to development must be fulfilled to equitably meet the needs of the present and future generations."