About 60 eels were relocated from the duck pond to the aviary pond in May last year after it was cleaned for the first time in nearly three decades.
After the deaths were discovered earlier this year, 22 eels were relocated to another, larger, duck pond.
A council spokesman at the time said the origin of the eels was unknown, though they had likely moved to the pond over land by their own means.
Invercargill City Council parks and recreation manager Caroline Rain said an examination was carried out and testing was performed via Massey University.
At the same time, water samples were tested — which came back as within potable water limits.
"Results from the examination were inconclusive and, following further advice from Massey, we have sent further samples to the Carruthers Institute."
After considering advice and guidance from mana whenua, the Ministry for Primary Industries, the Department of Conservation, Environment Southland, and Fish and Game, the population of eels in the aviary pond were relocated last week to the larger duck pond, she said.
"This pond offers deeper water, more space and a greater area for the eels to forage."
She said ICC rangers had been engaged in a multi-agency project to ensure the ongoing health and vitality of the Queens Park eel population.
Eight nets were set in the aviary pond by Fish and Game and were lifted last week to allow the relocation of the animals, which were mostly tuna kuwharuwharu (longfin).
All 22 were given a clean bill of health.
"It is encouraging to see a multi-agency approach to this work and to be reminded of the teams that surround us and help us continue our steadfast guardianship of the flora and fauna in our beautiful park," Mrs Rains said.
Environment Southland’s monitoring and evaluation teams have been assisting ICC staff with investigating the cause of the deaths, including monitoring dissolved oxygen in both ponds, and setting out fish traps to ascertain the presence or absence of fish in the pond.
