As the banks of the Parramatta River steadily rose and engulfed the Powerhouse Parramatta museum construction site on Tuesday afternoon, the state’s politicians in Macquarie Street were arguing about what constituted a one-in-100-year flood. Labor member of the legislative council, Walt Secord, grilled the new New South Wales arts minister, Ben Franklin, during question time. He attempted to table images and video from his phone to prove that the government’s controversial choice of site for the museum – one that will house many of the state’s most valuable and irreplaceable objects from 2025 onward – was misguided and foolhardy. Franklin, in response, said it was “patently absurd”, “far-fetched and utterly unreasonable” to suggest that the new museum, still in the very early stages of construction, would fail the City of Parramatta Council’s requirement to withstand a one‑in‑100‑year flood level, and that even in the case of a one-in-1,000 year flood, the ground floor of the museum would still remain half a metre above the flood waters. The man Franklin deposed, former arts minister Don Harwin, who drove the Powerhouse Parramatta project from its early days, told parliament he had viewed the same photos as Secord, and there was “a small amount of water over the grass at river level”. In images obtained by Guardian Australia, at least one flooded and abandoned car can be seen on what would normally be a road near the site on Tuesday, which saw 74mm of rain dumped on Parramatta in 24 hours, forcing the ferry jetty neighbouring the museum site to close. “The government, under the previous arts minister … said the museum was going to be built on a site with architectural changes that would mean it could withstand a one-in-1,000-year flood, but the opposition found that it has happened three times in three years,” Secord told parliament. “The minister has disputed that claim in the chamber, but I am relying on footage on the site posted by locals. Even as we were speaking, the entire area was completely inundated.” Local resident and freelance writer Alan Mascarenhas, who lives less than 500 metres from the museum site and captured images and video of the waterlogged site on Tuesday, said locals knew how prone the area was to regular flooding. “It only takes a couple of hours of sustained rain and the whole place is waterlogged,” he told Guardian Australia. “By yesterday afternoon the walkway down there was obliterated on all sides.” In February 2019, the NSW State Emergency Service’s senior manager for risk reduction and avoidance, George Jeoffreys, warned that Parramatta CBD residents could have as little as nine minutes warning of flash flooding from the river. Even in a best-case scenario, he said, the best warning the SES could give the public would be only two hours out from an event, making evacuation “difficult, if not impossible”. The $915m Powerhouse Parramatta project’s location on the banks of the flood-prone river has been an ongoing bone of contention between the state government, the opposition and a large group of local residents who are not opposed to a new major museum in the area, but are opposed to its current location. A month before the Parramatta River broke its banks and flooded the construction site in March last year, independent Parramatta-based flood management consultant Steven Molino told a state government inquiry the museum’s proposed one-in-1,000 year scenario meant that in the building’s predicted 100-year lifespan, there was a 10% chance that water was going to get into the building. “If it was an office building with just office furniture and carpets and everything, it wouldn’t matter – if it gets flooded you go down to Carpet Court and buy new carpet, you go down to Officeworks and buy new furniture – it’s all easily replaced,” he told the inquiry. Sign up to receive an email with the top stories from Guardian Australia every morning “But the information we have at present suggests that the electrical power supply will go out [during flooding] and the generators won’t operate the air conditioning system. So you’ll have a spike in humidity … and if you’ve got items in the collection made from paper, textiles, and even wood and some metals, [the museum’s] collection will deteriorate, be possibly irreparably damaged or even lost. “Now the decision might be made that the probability of this occurring is an acceptable risk, but no one has gone through that process.” On Wednesday Infrastructure NSW, the government body overseeing the museum’s construction, issued a statement, using an analysis of Tuesday’s flood data by engineering and flooding specialists Arup. The analysis concluded that “water from the heavy rain event on Tuesday 22 February would have peaked around 4 metres below the future Powerhouse Parramatta ground floor” and would have only entered the museum’s undercroft space, an area not used by the public. “Powerhouse Parramatta will be a very safe building for people to visit and for the collection to be exhibited,” the statement said. “The museum has been designed to withstand large and rare flood events.”