The ACT Coroner's Court is exploring the cause of the fire which swept through the heritage-listed New Acton Pavilion in June last year.

The blaze destroyed Flint, Parlour Wine Room and upstairs office space, as well as damaging the Diamant Hotel.

The court has previously heard the fire started in the exhaust ducts attached to a wood- and gas-fired pizza oven in Flint.

Donald Barr's company, Millennium Heating and Cooling Pty Ltd, was originally brought in to handle airconditioning for the site.

But Mr Barr yesterday told Anthony Williamson, counsel assisting the coroner, his role later expanded, at an informal onsite meeting with the developers, to include installing the exhaust ducts.

''If I had my time over again I wouldn't do it,'' he said. ''I don't do kitchen exhausts, they are by nature fairly peculiar types of work.''

The court heard Mr Barr had little or no experience in installing ductwork in commercial kitchens. He said he was unaware of Australian standards governing the installation of commercial kitchen ducts.

Mr Barr said he agreed to take on the job because he had a good relationship with a developer, who had been in a bind.

The court has previously heard a fire safeguard, a spray mechanism attached to the oven, was inactive at the time of the fire. Investigators later pulled material out of the ducting, prompting suggestions this could have acted as fuel.

Witnesses have said cleaning access panels marked in the design appeared to have been left off the ductwork itself. Mr Barr, who subcontracted its fabrication, on Wednesday said he was ''surprised'' the access panels were not included.

''If they're on the drawings that I actually had from the start, and they're not there [on the ducting], then ultimately my company installed that ductwork,'' he said.

''But I'm, you know, I'm surprised they weren't installed and I was surprised to find out they hadn't been installed at a later date.''

The hearing before Chief Coroner Lorraine Walker will continue on Thursday.