A foul odour and a deluge of seaweed prompted Wollongong City Council to close Wombarra ocean pool this week and over the long weekend.
The water turned a pretty light blue colour but the smell turned would-be swimmers away from Monday.
A sign at the beach on Thursday read "pool closed due to environmental factors", and "swim at own risk".
A Wollongong council spokeswoman said the rock pool was closed for maintenance work.
"While council's crews have emptied the pool of a significant amount of seaweed that washed into the pool last weekend, sea conditions have prevented the crews from safety being able to completely flush and clean the pool," she said.
"The movement of seaweed is a naturally-occurring phenomenon.
"Recent sea conditions continue to dump large amounts of seaweed into Wombarra Rock Pool, and as the seaweed is exposed to air and starts to breakdown it can create an unpleasant odour."
The spokeswoman said this week's volume of seaweed in the pool was so significant that it blocked the pool's intake valves and crews were required to get a pump and excavator into the pool to remove it.
"Our crews are closely monitoring the tide, seas and swell conditions and will carry out a full clean in the pool when it's safe to do so," she said.
"In the meantime, we're asking people to avoid the pool and beach area and enjoy our other ocean rock pools like those at Coledale or Coalcliff."
Despite the nice weather, the pool will remain closed over the Easter long weekend.
A lot of seaweed had also washed onto the beach.
Last year the council completed major structural improvements including the restoration and reconstruction of walls at Wombarra ocean pool.
New precast internal wall lining, extension of the existing sea wall and construction of a beach access ramp were also carried out late last year.
Council workers typically inspect all rock pools on a Monday and carry out maintenance on a needs basis throughout the week.
A four person crew works to maintain all the rock pools.