Tens of thousands of people remained stranded on Thursday evening while attempting to flee bushfire-ravaged areas of the south-east Australian coast – having earlier been urged to leave before the return of extreme and dangerous weather conditions.

The mass evacuation of communities in New South Wales and Victoria is among the largest ever emergency movements of people in Australia. The numbers fleeing the bushfire crisis remain unclear, but are expected to compare to the 60,000 people who were flown out of Darwin after Cyclone Tracy in 1974.

Visitors told to flee a vast evacuation area along the NSW south coast reported sitting in gridlock for up to 10 hours after responding to the order to evacuate, as further outbreaks of fire and sheer weight of traffic blocked escape routes north of Ulladulla and near Cooma in the Snowy Mountains.

Some political leaders find their natural authority in a crisis – not Scott Morrison

The prime minister, Scott Morrison, urged people to be patient, as he again deflected criticism about his government’s policies to address the causes of climate change.

“I know you can have kids in the car and there is anxiety and there is stress and the traffic is not moving quickly but the best thing to do – the best thing that helps those out there volunteering, out there trying to restore some order to these situations, is for everyone to be patient,” Morrison said.

On Thursday afternoon an angry protester told Morrison he should be “ashamed of himself” and that he had “left the country to burn” during a tour of the burnt out town of Cobargo.

Authorities in Victoria hold grave fears for 17 people missing across the state, and advised anyone who could do so to leave fire-affected places in the East Gippsland region.

New South Wales has declared a seven-day state of emergency before extreme conditions, including temperatures exceeding 40C, return to the south coast, Snowy Mountains and the outskirts of Sydney on Friday and Saturday.

Evacuation orders were issued to holidaymakers from Batemans Bay to the Victorian border and people were advised that two highway escape routes were opened – north to Sydney along the Princes Highway, and south, via the Monaro Highway, hooking back to Canberra through Cooma. But on Thursday evening both routes were at a standstill as the fire threat lingered, with tailbacks of up to 25km reported.

The northern route was blocked from about midday by a bushfire that closed the Princes Highway. Motorists heading north were advised to delay their trip or take shelter at Ulladulla. The road remained closed about 6pm. Motorists were reported abandoning their cars or heading to the south.