Canberra, Dec 4 : The state government of South Australia (SA) has imposed further border restrictions amid concerns about the Omicron Covid-19 variant. Premier Steven Marshall on Saturday announced that SA's borders will remain open to domestic travellers but those from the Covid-hit Australian Capital Territory (ACT), New South Wales (NSW) and Victoria must have a coronavirus test on arrival and isolate until it comes back negative. They will also be required to have a second test on day six if they are still in SA, Xinhua news agency reported. "We remain extraordinarily concerned about the Omicron threat," Marshall told reporters. "We don't know enough about the Omicron variant at this stage, so we are being very cautious." Police Commissioner Grant Stevens said the state's directions committee made a consensus decision not to impose a hard border closure but said "significant spread" of Omicron interstate would trigger stricter restrictions. "The best course of action at this point in time, in the absence of more detailed information, is to enhance our testing and surveillance, which is what we're doing," he said. As of Saturday morning there were 15 confirmed Omicron cases in Australia - 13 in NSW, one in the ACT and one in the Northern Territory (NT). The ACT's first case, which was confirmed on Friday, was fully vaccinated but has not travelled overseas recently, triggering an investigation into the source of the infection. On Saturday, Australia reported over 1,700 new Covid cases and 10 deaths 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 1,365 cases and nine deaths were reported. As of Friday, 92.8 per cent of Australians over 16 years of age had received one vaccine dose and 87.9 per cent have received their second dose, according to the Health Department. /IANS