The cost of flights from Sydney to Brisbane has skyrocketed as Queenslanders rush to return home before a border ban comes into effect on Saturday.

By Thursday, Virgin and Jetstar had sold out tickets on Sydney-Brisbane flights. For desperate Queenslanders, the only remaining option was a one-way ticket through Qantas – at a cost of $1600.

The soaring prices and demand come a day after Queensland Premier Annastacia Palaszcuk declared 31 local government areas, which make up all of Greater Sydney, COVID-19 hotspots.

The ban, which comes into effect from 1am on Saturday, means Queenslanders who can’t get home before then will have to fork out for two weeks’ mandatory hotel quarantine – at $2800 per adult.

On Wednesday, the state’s chief health officer, Dr Jeannette Young, urged Sunshine State residents in Sydney to head home as soon as possible.

“If you are in Sydney, please take this opportunity to come home to the safety of Queensland,” Dr Young said.

“If an area where you are staying or visiting is suddenly declared a hotspot while you are there, once you return to Queensland, you will have to quarantine for two weeks in a hotel at your own expense.”

Joseph Liu, who had planned to fly home at the weekend, heeded the warning.

“I’ve had to kind of rush it through,” he told the ABC.

“Try to get in there before the crazy stuff happens, you kind of get locked in here in NSW, so unfortunately I had to pay a bit extra and try to get on that flight.”

He said he visited several websites before he could secure a flight and saw seats completely book out within minutes.

“It was ridiculous … it was going crazy,” he said.

NSW reported 18 more coronavirus cases on Thursday, six of which remain a mystery.

One was an inmate at Sydney’s Parklea jail. The man – who had recently been in Victoria – was taken into custody in NSW on Sunday after being arrested for driving offences.

“CSNSW has established isolation hubs to safely manage COVID-19 positive inmates separate to other inmates,” a spokeswoman said.

“This is the first and only confirmed case of COVID-19 among correctional staff or inmates at any NSW correctional facility.”

Tweet from @NSWHealth

There are 89 cases associated with the Thai Rock Wetherill Park COVID cluster and 57 linked to the Crossroads Hotel in Casula.

Another 20 are connected to the south-west Sydney funeral cluster.

Meanwhile, tighter restrictions will soon be in place for gyms to ensure social distancing and strict hygiene as the number of NSW COVID-19 cases grows.

From Saturday, gyms across the state will need to register a COVID-safe plan and have a dedicated hygiene marshal on-site to ensure social distancing, equipment cleaning and hand sanitiser availability.

Fitness First gym in Kings Cross – near Sydney’s Potts Point coronavirus cluster, which numbers 11 people – confirmed on Tuesday a person who attended an exercise class on July 20 had tested positive.

Other cases in the area are linked to the Thai Rock restaurant in Potts Point – the sister restaurant of the Wetherill Park restaurant – and the nearby The Apollo restaurant. Two Queensland residents who dined at The Apollo were among three coronavirus cases confirmed in that state on Thursday.

There are confirmed COVID cases linked to the Thai Rock in Potts Point. Photo: AAP

Meanwhile, three Sydney schools were closed on Thursday after recording confirmed or possible coronavirus cases: Fort Street High School in Petersham, Freeman Catholic College in Bonnyrigg Heights and Mary Immaculate Catholic Primary school in Bossley Park.

The latter two have confirmed cases of COVID-19 and have undergone deep cleaning.

Everlearn Childcare Centre in Prestons has also closed due to a possible case in a child who might have attended while infectious.

Another case was detected in a patron at Frank’s Pizza Bar Restaurant in Camperdown on July 26. All patrons of the restaurant on that date have been told to seek testing and self-isolate for 14 days.

Other restrictions imposed on NSW residents include a ban on entering Western Australia without an exemption. Travel to South Australia and Tasmania is permitted, but people must quarantine upon arrival for 14 days.

Travel to the Northern Territory and the ACT is allowed but people coming from hotspots must quarantine for 14 days, while NSW residents who return from Victoria must self-isolate for 14 days upon return.

-with AAP