The third flight from Adelaide to be greeted by health officials since the explosion of new COVID-19 cases touched down at Canberra Airport just before 8.30am on Tuesday. A fourth will arrive on Tuesday afternoon bringing 110 people across both planes.

Most jurisdictions closed their borders with South Australia on Monday but the ACT and NSW remain open, with safety measures in place to screen passengers.

As passengers disembark the plane, ACT Health staff meet them and each is asked if they have been to any of the at-risk locations identified by SA Health.

Anyone with COVID-19 symptoms is told to isolate. Five people on flights that arrived on Monday night displaying symptoms are now in isolation.

A cluster linked to a quarantine hotel in Adelaide grew to 18 cases on Monday and restrictions were reimposed overnight in an attempt to reign in the spread of the virus, prompting some travellers to get in early should the situation worsen.

Margaret Arthur decided to push her flight forward two weeks late last night, scrambling to prepare to leave Adelaide's ballooning cluster for Canberra.

She would have arrived in the ACT next month for a family event, but is now preparing for a longer stay with her son and daughter-in-law.

"At nine o'clock last night I had to clean out the fridge and do all the things to tie down the house, and pack," she said.

She will likely spend Christmas in the ACT as her family grow concerned by the worsening situation in Adelaide.

"I was going to go back on December 11 but I don't know what will be happening now," she said.

Her son said if Adelaide went into lockdown, his mum would be isolated from friends and family.

When the pandemic hit in March, Ms Arthur spent seven months in the ACT and returned to her South Australian home just four weeks ago.

Ms Arthur said she was looking forward to one thing - a few more weeks out of the kitchen.

"I didn't cook anything for seven months! When I got home the food was awful," she laughed.

Eric Bardy planned to arrive in the ACT later this week, but made the last-minute decision to get in as soon as possible to visit his son, who he hasn't seen in five months.

He wasn't expecting to be greeted by health officials as he left the plane on Tuesday morning, and said the process was simple and took only a few minutes.

"I've been given no restrictions other than watch yourself and if over the next week you start to show symptoms make sure you get tested," he said.

Mr Bardy noticed a quick return to COVID-safe behaviour in his hometown, saying people became more aware of social distancing after months of complacency creeping in.

"As soon as it started - every one looks at Melbourne and nobody wants to be like that," he said.