The fire that caused chaotic scenes at the ACT's main public hospital on April 5 followed months of warning signs about the switchboard including electrical malfunctions dating back to at least September 2015.

Then health minister Simon Corbell told the Assembly at the time that the September 9, 2015 power outage was linked to a malfunction in a switchboard "which was overheating and was causing potential disruption to significant parts of the hospital".

The replacement of the switchboard was also later highlighted as an "extreme and a high risk" in as-yet-unreleased audit of hospital infrastructure, a legislative assembly committee heard last year.

Despite the warning signs, a contract for the switchboard replacement was not awarded until two days after the fire.

While investigations are still underway, The Canberra Times understands there was a mechanical failure in the switchboard that occurred as a contractor worked to isolate a circuit that supplied power to one of the hospital's lifts, in order to complete maintenance work on the lift.

The failure led to the small fire in the switchboard, which ultimately led to the evacuations and postponement of elective surgeries and outpatient services, while other patients were directed to Calvary Hospital in Bruce.

Electrical Trades Union ACT secretary Mick Koppie said the switchboard was "well past its use-by date", while an ACT Health confirmed that apart from routine maintenance, the switchboard had not been replaced since its installation "circa 1973".

"There's certainly been improvements or work that could have been done over the last 10 years, that if it had been completed, I believe the switchboard would not have had this problem," Mr Koppie said

He said another factor may have been the increased load put on the system due to air-conditioning over the summer period, which records indicate was one of the hottest summers in the ACT on record, a factor he believed was "probably the straw that broke the camel's back".

ACT Health has previously said an air-conditioning fault that days earlier led to smoke coming out of air-conditioning vents about two days before the fire was un-related.

But Mr Koppie said there had been a series of such faults in recent years, which he believed were due to delays in maintenance spending on the hospital's electrical infrastructure.

"To be honest, I certainly don't want to be a patient at the hospital when the lights go out," he said.

Tender documents show that despite ACT Health calling for "expressions of interest" in March last year for companies bid to replace the ageing main switchboard, no contracts were actually awarded until April 7 this year.

That $9.8 million contract was finally awarded to a "revised offer" from Shaw Building Group, more than six months after ACT Health received the company's original offer to complete the work in September last year, the documents show.

While an ACT Health spokeswoman said only that the offer was accepted on April 7 "at the conclusion of extensive contract negotiations" as part of "standard practice, but confirmed that other related work was not previously scheduled and was "expedited" as a result of the fire.

She said some $95 million was provided in the 2016-17 budget to maintain health assets, $23.5 million of which was allocated to the "electrical main switchboard program of works".

But answers to Questions on Notice in the ACT legislative assembly and budget documents from earlier years show the funds were originally allocated in the 2014-15 budget, but had been delayed until the 2016-17 budget.

The spokeswoman said ACT Health had since "undertaken urgent preliminary works to reduce the potential impact of electrical supply failure on clinical service delivery", including installing a new circuit breaker, a new portable disaster recovery electrical switchboard and a new generator supply.