Waitaki District Council personnel moved in on Thursday to dig out at least 1300cum of silt and sludge.
The pond is now drained and bereft of its resident perch, and a few longfin eel.
The cost of dealing with several decades of build-up was previously budgeted at $300,000.
In a statement, Waitaki District Council project manager Mark Burgher said the cleanout was expected to take about eight weeks.
‘‘Surveys of the mirror pond indicate that there is an average of 0.95m of silt, with depths varying from 50cm to 1.4m across the pond.
‘‘The total calculated volume of silt is around 1391cu m,’’ he said.
The silt is being trucked from the gardens via its western Chelmers St access to another site.
Mr Burgher said the Alps 2 Ocean trail and pathway, which enters Oamaru Public Gardens off Chelmers St and passes the ponds, is subject to temporary diversion.
‘‘This is the first de-silting of this pond since Waitaki District Council was formed in 1989, and council has no record of it having happened in the last 50 years.’’
The Otago Daily Times understands that historically, the gardens’ ponds were periodically and deliberately flushed when the Oamaru Creek was running high to ensure regular sludge clearance.
