Metservice forecaster Liz Walsh said it was still too early to say how badly Rotorua would be hit, but early indications were that Saturday would be the worst day and Sunday "isn't looking particularly great either".
She said early indications were for heavy rain and strong gales, possibly severe.
Relay for Life spokeswoman Sarah Fitzpatrick said they planned to shorten it to a one-day event starting at 2pm and finishing between 10pm and 11pm.
She said the changes were made because of safety concerns, and moving the relay to another date wasn't logistically possible.
"It's really hard. We've been watching the weather with baited breath but it looks like it keeps getting worse, not better."
Rotorua Walking Festival organiser Deryck Shaw said the event would still be on but they were taking a "watching brief" on the weather. Up to 1000 people are expected to take part.
He said the forecast was changing hour by hour and they would be waiting until closer to the event before making a decision.
The event wasn't weather dependant but if there were safety concerns for walkers then changes would have to be made.
One option was to move the day of walking in the forests to the Sunday if the weather was expected to be worse on the Saturday.
"We've had pretty good weather in the past. This is the 21st year and we've never cancelled it so it's not likely."
Tarawera Ultramarathon organisers were closely monitoring the weather but didn't think it would have too much affect. More than 850 runners are involved in the 60km, 85km and 100km solo and relay off-road run event, which starts in Rotorua and finishes in Kawerau.
"We're confident it's going to be fine for us," said media spokesman Graeme Simpson.
"It's going to be wet but the runners and team are prepared for that."
Pasifika Family Fun Day event organiser Paea Napa'a said they planned to go ahead in rain but would make a final decision closer to the time.
The Ministry of Civil Defence is warning people to be prepared ahead of the severe weather.
Civil Defence Emergency Management director John Hamilton said people should have emergency survival items and a household emergency plan as well as securing heavy objects and keep materials at hand for repairing windows.
He said farmers should know which paddocks were safe to move livestock away from floodwaters, landslides and power lines.
If warnings were issued Mr Hamilton said people should keep up to date with weather updates, secure items and store drinking water.
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