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New modelling from multiple sources suggests Sydney’s lockdown is still not tight enough to stop case numbers from growing.

As new locally acquired cases reached a record 177 in NSW on Wednesday, independent models from the University of Sydney, University of Western Australia and the federal government show the city needs tighter restrictions to control its outbreak.

Other models are still optimistic, predicting the outbreak could be brought under control in one to three months. But one team is so concerned about Delta spread it is now modelling stage five restrictions, where “nobody goes outside the door”.

The NSW government has been gradually tightening rules since the outbreak began last month, although it now plans to reopen the construction sector and allow year 12 students back to class.

NSW Premier Gladys Berejiklian has dismissed calls for a tougher, Victorian-style lockdown, similar to the one used by Melbourne at the height of its second wave of coronavirus.

Scientists often caution modelling should be taken with a substantial grain of salt, as a model is only as good as the data you put in.

But Sydney’s outbreak continues to grow, despite the city going into lockdown more than a month ago.

The city’s reproduction number based on current settings sat at 1.2 on Wednesday. A number above 1 means cases are continuing to grow.

“It is clear the current restrictions are not yet at the level where the suppression will be ensured in a rapid time frame. If you want to suppress it in a month, you need to do more stringent measures, that is clear,” said University of Sydney COVID-19 modeller Professor Mikhail Prokopenko.

University of Sydney COVID-19 modeller Professor Mikhail Prokopenko.

A study to be released by his team on Thursday that has not yet been peer-reviewed found marked improvements in social distancing since earlier this month has been slowing the growth of the outbreak.

But the Delta strain’s heightened infectiousness meant that was still not enough, Professor Prokopenko said.

At the University of Western Australia, Professor George Milne’s model also shows cases continuing to grow under current restrictions. His team is so concerned about Delta it is modelling “stage five” restrictions, where “nobody goes outside the door”.

“People are saying the current lockdown measures in NSW will put a cap on cases. Our results do not suggest that,” he said.

Delta is about 60 per cent more infectious than the Alpha variant of the virus, which in turn was perhaps 50 per cent more contagious than the version of the virus that first emerged in Wuhan.

“Delta has completely changed it, totally changed it,” he said.

The federal government’s modelling is done by a team of scientists around the country. It clearly shows the difficulty of containing Delta.

It uses a metric called transmission potential: the ability of the virus to spread. A number below 1 means cases will fall; above 1 indicates cases will rise.

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Were NSW facing the version of the virus that first left Wuhan, government modelling shows a transmission potential of 0.96, which would mean the epidemic would slowly shrink. With the Delta version of the virus, that rises to 1.75.

However, the Herald and The Age understands much of this data was collected before NSW’s restrictions were tightened.

Scientists close to the project estimate the number is lower now, but still above 1, and point out it varies widely across Sydney.

Separate models by the Burnet Institute and the University of Melbourne are more optimistic. They find stage three restrictions introduced in Sydney on July 9 would only stabilise case numbers, while the new stage four restrictions announced on Wednesday will drive them down quickly.

“Stage three restrictions together with the more infectious strain but improved contact tracing and some vaccine coverage, would not quite be enough,” said Dr Nick Scott, head of modelling and biostatistics at the Burnet Institute.

But the modellers now find themselves unsure if Sydney really has implemented stage four restrictions. Their modelling is based on Victorian-style restrictions which, at stage four included a curfew, the so-called “ring of steel” separating Melbourne from regional Victoria and mandatory masks inside and outside.

The NSW government is not allowing people to enter or leave the Greater Sydney area but does not include a curfew. Masks are mandatory indoors, but are only required outdoors if social distancing can’t be maintained.

”There are mixed signals coming from the NSW government. We were thinking this morning maybe they are in stage three. In terms of the adherence, how the lockdown is enforced,” said Dr Driss Ait Ouakrim, an epidemiologist and COVID-19 modeller at the University of Melbourne.

Facebook and Google mobility data, tracked by mobility analyst Rohan Byrne, suggests Sydneysiders are only slowly reducing how much they move around compared to sharp shutdowns in Melbourne.

“Sydney is currently at their lowest mobility yet, but as we learnt last year here in Melbourne, the comparison between partial or selective lockdowns and true universal lockdown could not be starker,” he said.

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