The Labour Department issued the amber heat stress warning three times on Friday.

The first alert was issued just before noon and lasted two hours, while the second and third warnings were issued at 2pm and 3.20pm and lasted for an hour.

Under non-binding guidelines for the amber signal, workers are advised to rest for 15 to 45 minutes an hour, depending on their physical workload.

Unionist lawmaker Lam Chun-sing said it was hard for both contractors and workers to react promptly to the changing signals.

"Of course it is not an ideal operation. Employers and employees are so confused," he said.

"It’s difficult for the workers to check their mobile phones all the time to see whether there is a heat stress warning, because they focus on their work."

Lam said many contractors and supervisors are still struggling to categorise how labour intensive the work of various employees is, in order to ascertain what steps they should take when alerts have been issued.

He also urged the Labour Department to check how the guidelines are being implemented on construction sites.