Alarm is growing for the impact of Cyclone Alfred, amid fears it could make landfall as a category three system and at high tide.

Widespread emergency alerts were issued for Alfred on Wednesday from Double Island Point in Queensland to as far south as Yamba in northern NSW, south of Byron Bay.

It is now expected to make landfall in the early hours of Friday (local time), between Queensland’s Sunshine Coast and Gold Coast.

It will bring heavy rain, flash flooding, destructive winds and storm surges at its height, with wild weather already picking up on Wednesday afternoon.

Queensland’s south-east is in the firing line, with about 20,000 properties expected to be affected by flooding in the Brisbane area alone. A further 6000 on the Gold Coast are also at risk.

Queensland Police were door-knocking homes in the affected areas on Wednesday.

Bureau of Meteorology modelling on Wednesday morning suggested the category two system could make landfall over the Redlands, south-east of Brisbane, before veering north and passing over the capital’s CBD in an unprecedented direct hit.

“The possibility of the system reaching a low-end category-three strength before making landfall cannot be ruled out, but remains a low risk,” a BOM report said.

Winds gusts and gales of 100km/h-130km/h are possible, followed by hours of rain that bring as much as 700 millimetres by Saturday.

Brisbane Lord Mayor Adrian Schrinner said there was a slim, but real, chance of that much rain.

“If that eventuates, then we’re talking about more like a 2022 flood situation,” he told ABC Radio.

BOM senior meteorologist, Miriam Bradbury, said that regardless of where the cyclone finally crossed, the effects could be severe.

“The real thing that people need to keep their minds focused on is that point of crossing is just where the eye of the system is going to move,” she told ABC News.

“The impacts will be much more widespread and may impact any areas within those warnings that we do have currently, and certainly, that tropical cyclone warning stretches from Double Island Point (South K’Gari/ Fraser Island) all the way down to areas just to the north of Grafton.”

On Wednesday, millions of people were finalising preparations before south-east Queensland shut down later in the day. Public transport will come to a halt with offices and schools closed across the region from Wednesday night.

The northern rivers region of NSW is also bracing, with more than 100 schools closed and locals urged not to panic-buy.

“Thursday is the day to act because Thursday evening and Friday morning we’re expecting the storm to approach and cross into NSW,” NSW Premier Chris Minns told ABC News.

Evacuation centres are set to open across Brisbane, the Gold Coast and the northern rivers.

NSW State Emergency Service Deputy Commissioner Deb Platts said the slow approach of Alfred had allowed time to prepare.

“We do have modelling that will predict that we all have moderate to major flooding across many of our catchments, right from Taree all the way up to the border with Queensland,” she said.

“At the moment, we are very concerned.”

In Brisbane on Wednesday, Prime Minister Anthony Albanese said  a Sikorsky s61 had been sent to Coffs Harbour and a US 60 Black Hawk would soon fly to Bundaberg to be on standby to assist with emergency operations of needed.

“Prepare, take this seriously, this is a rare event,” he said.

Albanese said the Australian Government Disaster Response Plan had been activated and a crisis coordination team and liaison officers sent out.

“We’ve convened the National Coordination Mechanism to assist with the coordination of the Commonwealth government response,” he said.

The National Emergency Management Agency was to meet later on Wednesday, bringing together supermarkets, banking, freight companies, telcos and insurance companies.

Flights are expected to be affected but Brisbane Airport on Wednesday said it was operating as normal while monitoring Alfred’s approach.

-with AAP