Department of Health reports show Bunbury Health Campus has had the longest patient wait times of any West Australian regional hospital consistently every month from January to June.
The WA Department of Health collects data each month from 24 hospitals across the state, with 14, including Bunbury, falling under the WA Country Health Service (WACHS).
All hospitals adhere to the four-hour rule, which was introduced in 2009, where medical staff look to treat emergency department patients within this time period.
Of the 14 WACHS hospitals, Bunbury has recorded the lowest percentage of patients being seen within four hours every month from January to June.
The best performance was in January, when 72.5 per cent of patients were seen within the desired period. The worst was in June, when 68.1 per cent were treated within four hours.
None of the other 13 WAHCS hospitals dropped below the 80 per cent mark in the first half of 2019, with the next lowest performance recorded at Kalgoorlie Hospital in June, at 80.6 per cent.
All of the 2019 Bunbury results were worse than the equivalent month in 2018, with an 8.1 per cent drop in performance in June 2019 compared to 12 months prior.
However, in all six months, the number of presentations increased from the 2018 figures, with 3602 patients in March the most for this year.
A WACHS spokeswoman said state government funding would help to upgrade the Bunbury hospital's infrastructure.
"We know presentations at Bunbury Health Campus ED have increased and an early start to the flu season placed further demand on our facility," she said.
"Our number one priority will always be patient safety and we closely monitor activity and implement strategies to manage the increased demand.
"We aren't shying away from the fact patient numbers and patient acuity have increased and welcome the government's commitment of more than $22 million to the facility's redevelopment.
"This funding will allow us to plan into the future and importantly, allow us to undertake the first phase of redevelopment including reconfiguration of our ED fastrack area and the establishment of an acute medical assessment unit.
"We remain committed to treating patients at Bunbury Health Campus in a timely manner and evolving our services to cater for the growing needs of Bunbury and the South West."
West Australian acting health minister Ben Wyatt said future upgrades were "crucial."
"The McGowan government last week announced an additional $11.8 million for the redevelopment of Bunbury Hospital - this brings the total commitment to almost $23 million and is recognition of the increasing demands on health services at the hospital," he said.
"This funding injection will fast track crucial upgrades including redevelopment of the Emergency Department to help the hospital meet the growing needs of the area.
"This is the most severe and sustained flu season in WA since records began and since January the number of people attending Bunbury Hospital ED with flu like symptoms has increased dramatically. While some people may have to wait a little longer those needing urgent medical care are prioritised and seen first.
"The Bunbury Step up Step Down centre is due to open early next year offering short-term residential care for people with mental health issues which will also help ease pressure at the busy Bunbury Hospital Emergency Department where we know mental health presentations are also an issue."