Manukau Harbour only doubtful after heavy rainfall

Manukau Harbour is a safe place for swimming except after heavy rainfall, according to new research.

Applied science student Sarah MacCulloch has recently completed her thesis on the area and has found the quality of the water has substantially improved since the oxidation ponds were removed in 2001.

"From what I've found it is safe to swim except after heavy rainfall," said the Auckland University of Technology student.

Ms MacCulloch said rain caused increased land run-off and overflows from the local wastewater treatment plant to flow into the harbour, which is the case for many beaches.

She said there had been a huge build-up of fine clay and silt sediment around the oxidation ponds and nearby Nga Kuia e Toru reef, off Puketutu Island.

"The removal of the ponds resulted in greater exposure to tidal flows," Ms MacCulloch said. "In turn this led to a substantial decrease in the amount of fine clay and silt sediment around the reef.

"When I first started going to my site I was thigh deep in mud. Now it's only ankle deep."

Watercare Services communications manager Mark McLauchlan said his company's research supported Ms MacCulloch's findings.

He would not say categorically that the area was safe for swimming, preferring to await the completion of studies in about 18 months. However, the indicators were very good.

"It has been noted by fishermen that this year has been a real boom for fish," he said. "And anecdotal evidence says there is an increase in migratory wading birds, so feeding must be good."

The oxidation ponds were built in 1960 and covered 500ha.