Chief Executive John Lee said "stern and serious" measures were required in response to the discharge of wastewater from a nuclear power plant in Japan.

He also said there's no guarantee the process could be free of risk or human error.

Speaking after attending a consultation session on his Policy Address in Sha Tin on Sunday, Lee said nuclear-contaminated water that had contact with reactors could bring about more than 30 types of radioactive substances.

"We also have to be careful of the long-term risk because of the accumulating effect of radioactive substances being poured into the sea, and the accumulation – we are talking about a period of almost 30 years or even over," Lee said.

Hong Kong imposed restrictions on aquatic product imports from 10 Japanese prefectures on Thursday as Tokyo started releasing wastewater from the tsunami-stricken Fukushima nuclear power plant, a process the CE described as "unprecedented".

"Nobody can tell that it will be risk-free. The other thing is, there is yet to see in the world that any system or any method will be risk-free or free of human errors," Lee said.

"We have to ensure that for a system which will run for such a long period, the reliability of the equipment, the maintenance of the system, and the possibility of a human error in an exercise that will last for over 30 years. That is why we have to take stern and serious measures to ensure food safety and the health of our people."

Secretary for Environment and Ecology, Tse Chin-wan, said authorities from outside Japan should be allowed to monitor the wastewater release from Fukushima.

He also said Tokyo should not only focus on tritium when releasing monitoring information, as the radioactive substance found in wastewater is not the only thing to worry about.

"Although Japan said the wastewater is treated and has met standards before being discharged, we are most concerned about whether the process is done well... It's clearly not enough to only focus on tritium levels, Japan needs to publish other data as well," he told a TVB programme.

"We need international participation, and it is necessary to have an open and transparent monitoring mechanism. We, the affected areas, should also take part in monitoring to build confidence."