Another has warned the highly infectious variant "resets the clock" on vaccines, after data showed it rendered a two-dose course largely ineffective.

Omicron is tearing through Europe, prompting a pre-Christmas return to hardline restrictions in a host of countries, and University of South Australia biostatistics expert Adrian Esterman says its trajectory should be a warning to Australian leaders over abruptly lifting restrictions.

NSW's seven-day case average is on the march again - the state recorded another 804 infections on Tuesday - though authorities insist their reopening schedule will not be blown off course.

Professor Esterman believes a major wave likely to last several months is under way, but a number of health measures - mask wearing, social distancing, hand hygiene - had kept outbreaks in check without "killing" business.

He said it "makes no sense" to relax those measures in NSW and Victoria while Omicron grows roots in the community, while mask mandates should return for settings like retail in the ACT.

"It gets a little bit worrying when politicians suddenly decide to relax restrictions when we're just about to get a new variant hit us," he said.

"At this stage, we simply don't know how nasty it is."

Australian authorities suggested early data showed Omicron producing less severe symptoms than its predecessors.

But Professor Esterman said firm data on Omicron's severity is likely weeks away, with experts currently relying on "anecdotal" data from South Africa. That is complicated by the fact the average South African is younger and less likely to suffer severe symptoms.

"To just assume it's going to be milder I think is a big mistake," he said.

The UK suffered its first death from Omicron on Monday, the same day it recorded nearly 55,000 cases, and the variant now accounts for one-fifth of infections in the country. Downing Street has scrambled to neuter its spread, slashing wait times for boosters from six to three months and re-imposing a host of restrictions.

But Professor Esterman said, unlike many countries in Europe, Australia never completely eased restrictions and is not reacting to Omicron from a standing start.

And as hospitalisations flatlined in the UK despite raising case numbers, chair of epidemiology at Deakin University Catherine Bennett played down fears for Australia.

Professor Bennett said the UK was coming off a "really high base" before the recent spike, with case numbers hovering at around 35,000 each day, roughly equivalent to 3,500 in Victoria.

But with Victoria treating more than 360 patients in hospital on Tuesday, compared to 198 in NSW, she said Delta remains the "major concern".

"There's something going on in Victoria where we are clearly getting more Delta infection," she said.

"While we find it hard to predict what Omicron is going to do, at least at this stage, it seems the risk isn't as immediate on the hospital load."

But having avoided high infection rates seen overseas, and with new data suggesting two vaccine doses offered limited protection against Omicron, Professor Bennett warned Australia had "neither natural immunity [nor] a very well targeted vaccine immunity".

Third doses were showing signs of bolstering protection against Omicron, and the recommended six month gap between second and third doses has been cut by a month after the strain arrived in Australia.

A six-month gap between second and third doses provides peak efficacy against previous variants, though that has been cut by a month after Omicron arrived in Australia.

Professor Bennett said emerging data could prompt authorities to cut that timeframe even further, but keeping a lid on infections in the interim may provide breathing space.

"With Delta, we knew we had a waning problem. Omicron just resets the clock," she said.

"If you just give people another shot now, it might not do a whole lot more ... [But] if we can keep infections in check and hold out [for] five months, then we might get the best of both worlds."

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But keeping maintaining a holding pattern may prove particularly difficult over Christmas period, with regular gatherings and families crossing state lines.

Professor Bennett warned families to be vigilant in the coming weeks, with gatherings held outdoors where possible, rapid antigen tests used, and people with symptoms staying at home.

"If we do all those things, then hopefully we won't see this take off," she said.

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