Bondi Beach could be closed for up to 36 hours while tests are conducted on the algal bloom that stained the water a red colour this morning.

A spokeswoman for Sydney Water said a maintenance team, including a water quality expert, arrived at Bondi at 11am to carry out extensive tests on site, plus take samples to be tested in laboratories.

''We think it is looking very likely that it is a blue-green algal bloom,'' she said.

Algal blooms often appear in hot and humid weather, such as experienced in Sydney during the past three days. An expert from the NSW Department of Water said blue-green algae is often brown, red or pink in colour because of photosynthetic pigment, which gives a brown, red or pink colour to the cells.