Tighter ‘Stage 4’ restrictions will likely be needed to control Delta variant outbreak in NSW, new modelling shows

Burnet Institute

12 July, 2021

New modelling, developed by Burnet Institute, shows if NSW is to control its current Delta variant outbreak of COVID-19 then stricter Stage 4 restrictions, similar to those introduced in Victoria’s second wave in 2020, will be needed. With the urgency of rising cases in NSW, Burnet is releasing this non-peer reviewed modelling.

NSW’s original stay-at-home orders and updated restrictions (introduced 9 July) combined with contract tracing, have averted a significant number of infections, hospitalisations and deaths.

However, the COVASIM mathematical modelling also suggests the impact of the current restrictions in NSW have not been effective enough to control the outbreak.

In summary, the modelling shows:

  • Giving up the lockdown in Greater Sydney would be catastrophic.
  • NSW’s updated restrictions (introduced 9 July) are approximately equivalent to Victoria’s Stage 3+masks restrictions. Our simulation shows Stage 3 + masks restrictions would prevent daily case numbers from increasing further, but are unlikely to be sufficient to eliminate community transmission in an acceptable time frame.
  • If Stage 4 restrictions were applied now the epidemic curve would decline sharply. It is difficult to estimate the time to return diagnoses from current levels to a 7-day average of <5 cases per day but this would be likely in the order of a month.
  • Even if density restrictions are maintained for an entire year, without additional restrictions the model projects around 12x as many deaths as influenza deaths in 2017 – one of the worst years on record.
  • Consistent with other settings around the world, if the aggressive suppression strategy is ended now, a subsequent long lockdown would almost certainly be unavoidable.
  • If restrictions are only reintroduced once ICU capacity is reached, a harder lockdown would be necessary to bring cases down, because contact tracing will be more difficult. Also, it would likely need to last longer than if a harder lockdown was introduced immediately.

Burnet Institute Deputy Director and leading infectious diseases and public health specialist, Professor Margaret Hellard AM said that the modelling estimated whether the ‘Stage 3’ and ‘Stage 4’ restriction levels used in the Victorian second wave in 2020 would be sufficient to control the current Delta outbreak in NSW.

“There is little doubt that whilst the current strategy of NSW Health has impacted on the course of the epidemic, including their stay-at-home measures, mask policy and trace and track interventions, our modelling indicates more is needed to stop this infectious Delta outbreak,” Professor Hellard said.

“NSW’s current restrictions are approximately equivalent to Victoria’s Stage 3+masks restrictions, so the good news is they still have ‘room to move’ in tightening the public health response. Contact tracing has been beneficial and remains critical.

“It is important to recognise that issuing public health orders do not necessarily translate directly to optimal uptake of those orders. Community engagement should be enhanced and form the backbone of adherence measures. Where necessary individuals and communities should be supported to help them comply and follow the restrictions.”

Burnet’s Director and CEO, Professor Brendan Crabb AC said there was benefit in “going hard, going fast” in any pandemic response but NSW still had public health levers they could use.

“Introducing more stringent measures like Victoria’s Stage 4-style restrictions would likely bring the outbreak under control and reduce the length of lockdown,” Professor Crabb said.

“Although it is difficult to measure the direct benefit of sending strong signals to the community about the seriousness of the current situation in Sydney, measures such as curfew and masks outdoors should also be seriously considered as these were a part of Melbourne’s Stage 4 restrictions.”

Burnet Institute Head of Modelling, Dr Nick Scott said: “The model estimates the impact of policy packages based on the experience of Melbourne’s second wave and has been continually updated to account for the increased transmissibility of the Delta strain, timely and efficient contact tracing and QR scanning, and the use of masks.”

“We modelled an outbreak of comparable size to what is currently occurring in NSW and assessed the impact that the policy settings previously used by Victoria would likely have.”

COVASIM is an agent-based COVID-19 model developed by Burnet Institute and Institute for Disease Modelling in the USA. It provides governments with more specific and precise data to inform their COVID-19 responses. More information at

Contact Details

For more information in relation to this news article, please contact:

Professor Margaret Hellard AM

Deputy Director, Programs; Adjunct Professor, Monash University, DEPM.




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