The bacteria was detected on Wednesday last week but warning signs were not put in place until Tuesday.
ORC resource science manager Dean Olsen has admitted the error.
''QLDC and Public Health South were not notified immediately following the high reading on 11 December.
''This result was received on Wednesday last week but was not followed up immediately, as it should have been. As a result, a swimming advisory did not occur ... and warning signs were not put up until yesterday [Tuesday].''
The signs were installed by Queenstown Lakes District Council after it offered support to its regional counterpart.
Results showed a reading of 921 E. coli per 100ml - national guidelines say anything over 550 E. coli per 100ml ''poses a health risk''.
Communications adviser Jimmy Sygrove raised questions over timing.
''It is ORC's responsibility to put signs up and notify the public. ORC oversee the health of our waterways, lakes and rivers and carry out testing. We weren't made aware of the high readings until Monday afternoon.
''Any delay in notifying the public in this sort of situation is obviously a concern.''
He was not the only concerned party.
Mother of two Cath Hudson is ''furious'', and accused ORC of being ''negligent''.
''It is not on. Especially in the first few days of school holidays, on a beautiful sunny day, they will have known.''
Her youngsters were swimming with friends, one of whom was flying to England yesterday to visit elderly grandparents.
''If she had got sick she wouldn't have been able to fly and if she is going to get sick ... then her first week spending time with her grandparents is potentially ruined because someone has been negligent.''
Yesterday afternoon, after further tests, ORC said the lake was safe for swimming again.
A water sample collected from Mill Creek at the same time is being tested to determine the likely source.
It follows a case in Glenorchy last week when people were advised to boil water after the bacteria was detected in the township's water supply.
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