A former Observatory chief, Lam Chiu-ying, said on Sunday the government should focus on developing flat land in the New Territories before going for other options.

He was responding to the Chief Executive John Lee, who vowed on Saturday to explore all options to find land for homes, including using parts of the green belt.

But Lam urged officials to take an integrated view of land supply, rather than studying individual sites, adding it's better to first develop brownfield sites and abandoned farmland near villages.

"If you build new housing estates on flat land, it will be cheaper and those sites are usually closer to existing roads. It will be cheaper; it will make travelling for the residents much easier," he told a Commercial Radio show.

"There's no reason why we should ignore those existing sites that are accessible by transport but go to remote places where you need to invest a lot to ensure people can travel cheaply."

Discussing the recent heatwave affecting the SAR, the former director of the Observatory said Hong Kong people don't seem to be aware that even more hot weather is on the way in future, and that increasing the use of air conditioning to combat that will only worsen climate change.

He said people should try to minimise their use of air conditioners and improve natural ventilation.

Lam added that glass-clad buildings are also exacerbating the problem, raising temperatures inside the blocks, which means an even heavier loading on air conditioners.

"The government should take measures to stop this trend of having more glass on the shell of buildings. This will involve critical examinations on how glass is making the problem worse. I would think this is something the government must do very quickly, otherwise all of us will be living in glass cages in the future and spend a lot of money in air conditioning and making climate change worse," he said.

Observatory data shows that Hong Kong has seen 23 nights this month where temperatures haven't dipped below 28 degrees, the highest since records began.

Speaking on the same radio show, the deputy director of the Society for Community Organisation Sze Lai-shan said people living in subdivided flats have faced temperatures of 40 degrees or more, and low income families living there would see their utility bills soar.

"The hot weather affects those living in cage homes and subdivided flats a lot because it's too hot and they can't sleep. If they have air conditioners, they will turn it on or the fan. So they need to pay more for electricity and water," she said.

Sze urged the government to discuss how to install electricity and water meters in subdivided flats to prevent overcharging by landlords.

She also said the administration should consider providing subsidies to grassroot workers, such as cleaners, for them to buy water or mini-fans to keep them cool during very hot weather.