All aboard the coronavirus express! Horrifying image shows packed Sydney bus with almost no one wearing face masks as NSW records an alarming number of fresh cases

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A shocking photo of maskless, jam-packed commuters has been posted online as Premier Gladys Berejiklian warns of harsher restrictions for NSW.

The picture was taken by a passenger on board the M52 bus at Ermington in Sydney's north during the morning peak hour rush.

The woman who posted the image said 'social distancing' was impossible on the crowded commute.

This photo shows maskless, jam-packed commuters on board the M52 bus at Ermington in Sydney's north

'There wasn't any social distancing at all, the majority of people weren't wearing masks. It was shocking,' Hanna told 9News.

Under coronavirus protocols, a total of 24 passengers is the guideline for a two-door Sydney bus.

But Hanna said there were at least 35 people on the bus when she got on.

'I get it when they say don't take the bus if it's too full but I need to get to work,' Hanna said.

'There is only one bus that goes to my area so if I don't take this bus then the next one is in 15 minutes away and there is no guarantee it won't be as crowded.'

NSW recorded 18 new COVID-19 cases in the past 24-hours including two more infections linked to the Tangara School for Girls in Cherrybrook.

The origin of the cluster has now grown to 19 cases and the source of the outbreak still remains under unknown.

There are reports a student retreat was responsible for an outbreak at the school, which has grown to 19 cases (pictured) 

Premier Berejiklian said the mystery transmission means the state will remain on 'high alert'.

'We know the rules we have put in place, the advice we have provided works so long as it is applied across the board,' she said.

'Whether it is a school extracurricular activity, which shouldn't occur, whether it's the recommendation for people to wear masks when they can't guarantee social distancing on public transport or in supermarkets, or whether it's the way in which we've asked businesses to approach a COVID-safe environment, compliance is absolutely critical.'

The Premier said although there had been an increase in the uptake of compliance including the wearing of masks, she warned there still needs to be even more.

'If we don't see a greater uptake in the next little while, we will consider further measures,' she said.

'While numbers have remained stable in NSW we can't be assured of that moving forward.' 

Health Minister Brad Hazzard reiterated the Premier's comments on Wednesday. 

'The bottom line is: masks,' he said.

'We all need to be reminded that this virus has not gone away ... Treat yourself and treat everybody else as if you have got the virus.' 

Premier Gladys Berejiklian said If we don't see a greater uptake on mask there may be more restrictions