The lake last night was edging closer to the "possible flood" level of 311.3m.
It passed the first flood warning level of 310.8m on Monday afternoon and reached 311.114m by 2pm yesterday.
The MetService has forecast another 100mm of rain for the area over the next 24 hours.
The district marked the 10th anniversary of the devastating 1999 floods in November, when the level of the lake hit a record 312.8m.
The total insurance cost of that flood was $46 million and left many waterfront businesses unable to get insurance.
Otago Regional Council environmental information and science director John Threlfall said the council had done modelling on the effects of the predicted rain, and Lake Wakatipu could reach 311.65m tomorrow.
Queenstown Lakes Mayor Clive Geddes said the council had made contact with retailers in the 1999 flood zone to "dust off flood plans, empty grease traps and consider dry land options for stock".
There was a likelihood low-level flooding would occur on roads and reserves today or tomorrow, he said.
Yesterday, businesses raised shelves, emptied premises and stored goods in the second storey of buildings or off site.
Among those were Vesta Design Store and Cafe, in the historic Williams Cottage in Marine Pde; Country Road in the Mall; Pumpkin Patch, in Rees St; and Patagonia Chocolates and MacPac, both on the Queenstown waterfront.
Country Road manager Jeff Hayes said he was not taking any chances.
"We've been told the lake is close to flood level and more rain is heading this way.
"We're not taking any risks."
Kawarau Jet operations manager Fraser Gordon said the company's primary concern was with the underwater observatory, which might have to be sealed.
"It's probably got 25cm to go before it starts coming in."
Kawarau Jet was still operating, but leaving from Frankton Marina, for safety reasons, Mr Gordon said.
Mr Threlfall said the proactive attitude was "probably wise".
The problem was the rain forecast for today, he said.
There was also a "probability" the lake would not peak tomorrow, but continue to rise on Saturday.
Lake Wanaka was also on the rise, predicted to surpass the first flood warning of 279.4m tomorrow and to reach 279.6m.
"But possible flooding is at 280m, so there's still half a metre to go," Mr Threlfall said.
"We're not concerned about Wanaka. It's lagging behind a bit, which is fine.
"Our immediate concerns are just with Wakatipu."
Mr Geddes said more would be known today, but if "more significant measures" were required, for example sandbagging, retailers would be given plenty of notice.
The ORC and QLDC had adopted a "watching brief".
At Kingston yesterday the lake appeared to be at its highest level since the 1999 flood.
Community Association secretary Margaret Gibson said the water was up to the base of the boat ramp, which was used to take TSS Earnslaw out of the lake.
The Te Anau glow worm caves had flooded yesterday afternoon and Real Journeys staff had begun sandbagging their basement at Manapouri as a precaution against flooding.
