He said orders were stopped being taken in early February for Gib producers to dispatch the backlog and work out a new allocation system, based on a merchant's previous year's sales.
Gib and the companies behind that product had been great at meeting builders' needs and creating a product that had improved over time, O'Leary said.
"Gib has been great at listening to the industry for years, streamlining their own system, creating a bespoke, easy-to-use design software, and educating builders, designers, and councils in how to use this."
But as a result of that, they had carved out a position as the dominant supplier in New Zealand and muscled out opposition products, O'Leary added.
Winstone Wallboards has a 94 per cent market share of Gib board in New Zealand.
The supply shortfall has led to Gib being sold online with mark-ups of more than 1000 per cent.
An online seller in Ohakune was selling a stack of standard Gib boards at a price of $410 each.
They are typically sold by merchants at $27.62 a board, meaning those online boards were being sold with a mark up of 1384 per cent.
As well as the shortfall in Gib, there is a boom in housing construction going on at the moment around the country.
StatsNZ said between March 2021 and March 2022, 50,858 new building consents were granted for homes.
That's a 24 per cent increase on the year before and is also the first time the annual number of building consents granted has passed 50,000.
A total of 5303 building consents were granted across the month of March 2022, which is also the highest monthly number on record, StatsNZ said.
"We're in the biggest building boom since after World War II and the demand is still there, so from a demand-side, we are not losing that demand," said the manager of Placemakers Whanganui & Ohakune, Simon Richman.
Richman said some Whanganui merchants were restricting the amount of Gib, a customer could purchase at one time to prevent bulk buying.
This would also help merchants maintain a supply of stock as there continued to be a national shortage, he said.
"We can't order the amount of Gib that we want, and we have to plan far in advance for stock orders," Richman said.
Early in March the general manager of Gib producers Winstone Wallboards, David Thomas announced Gib would be altering its order model so boards were sold to customers based on their previous ordering.
This meant merchants would receive a set volume of plasterboards each month.
Thomas said the forecasted monthly supply has already been supplied for July through to September.
Richman said this new model wouldn't necessarily be a fix to the shortage, but the spread of supply would help some merchants and builders in smaller areas while hurting others.
"Some people have been massively impacted by the shortage, and others not impacted at all, so they (Gib) would rather spread the hurt across the board."
Last week Placemakers also advised their customers they should get their Gib orders for July in now as the amount they were able to order was strictly controlled.
Placemakers is the retail arm of Fletcher Building, which also owns Winstone Wallboards, the producers of Gib.
However, Richman said Placemakers got no preferential treatment over other merchants.
"The Fletcher Group is adamant that they do not treat their subsidiary companies above the other merchants," he said.
In the long-term, Thomas also said the on-allocation model will be in place until at least June next year when a new production plant for Gib in Tauranga is scheduled to be operational.
