Canberra, Dec 2 : The state government of South Australia (SA) has tightened its border restrictions in response to the latest coronavirus outbreak. SA on Thursday reported 18 new Covid-19 cases, 16 of which have been linked to a school reunion event on Saturday night while the remaining two are believed to have become infected interstate. Authorities have not identified the source of the school reunion outbreak but believe it is someone who brought coronavirus into SA after it opened its borders to travellers from Covid-hit New South Wales (NSW), Victoria and the Australian Capital Territory (ACT) on November 23. In response to the outbreak and concerns over the Omicron variant, SA Premier Steven Marshall announced that all travellers from NSW must now be tested on arrival in SA, Xinhua news agency reported. The state has also increased the quarantine time for international arrivals from seven days to 14 days. Marshall said he was not surprised by the spike in case numbers after opening the border but that the Omicron situation must be monitored "extraordinarily carefully." On Thursday, Australia reported more than 1,700 new Covid cases as the country continues to battle the third wave of infections. The majority of new cases were in Victoria, the country's second-most populous state with Melbourne as the capital city, where 1,418 locally-acquired cases and 10 deaths were reported. The number of confirmed Omicron cases in the country has increased to eight after a recently returned international traveller tested positive in NSW. As of Wednesday 92.6 per cent Australians aged 16 and above have received one vaccine dose and 87.4 per cent have received their second dose, according to the Department of Health. /IANS