Chief executive Peter Bodeker said the council's environmental officers had collected water samples at several points along the creek, near Speargrass Flat, last week and again yesterday, both up and downstream of where the dead fish were found.
A report was received by Fish & Game on June 2, and the ORC notified. More dead fish were found during a site visit on June 7.
The water samples were being analysed by two laboratories to establish the presence of pesticides, or other chemicals, and heavy metals, as well as the overall quality of the water in the creek and an adjacent wetland.
Results from that analysis were expected in a week, while results of autopsies on two of the dead fish - sent to the Cawthron Institute in Nelson- were expected in a fortnight.
Mr Bodeker said his staff in Central Otago were liaising with the Queenstown Lakes District Council's compliance staff and Otago Fish & Game.
''So far, there is no obvious cause that anyone can point to.
''There is no construction site nearby and no signs of the sort of land disturbance or other activity that might generate pollutants.
''We are keeping an open mind about the cause at this stage and not ruling anything in or out.
''We are very keen to hear from anyone who has noticed any unusual activity along Mill Creek in the Speargrass Flat area.''
The regional council regularly monitors water quality in Mill Creek, as it is one of the significant inflows of Lake Hayes.
Mr Bodeker said calling the ORC's 24-hour pollution number - 0800 800 033 - was the easiest way for people to report water quality issues anywhere in Otago.
