More than £3.3 billion of Irish NAMA loans are secured on assets in Northern Ireland, a conference heard.
Most of the debt involves undeveloped land and some may go back to farmland, the so-called bad bank's Northern Ireland representative Peter Stewart told the Belfast audience.
He said while the agency did not have to sell immediately, it had to operate on a 10-year timescale.
"Even over such a period of time it is likely that we will see what was previously viewed as potential development land being sold to go back to farm land," he said. "For those builders, developers and also land traders and speculators who got caught up in the frenzy, unfortunately there is going to be financial pain."
The National Assets Management Agency was set up by the Irish government to help save Dublin-based banks which have been crippled by the property crash.
Although NAMA is thought of as a way of removing "toxic" loans from the banks it is also taking over good or "performing" development loans - those which are being repaid and earning money.
Mr Stewart, chairman of NAMA's Northern Ireland advisory committee, said there was not a huge over-supply of residential properties in Northern Ireland comparable to that in the Republic. He added the investment property market had been worth almost £1 billion at its peak in 2007.
"It is clear that the NAMA-secured supply side of the market is significant in terms of normal turnover levels and there will be challenges in realising these assets," he said.
He said NAMA will be prepared to consider financing proposals that could involve joint ventures, shared security, phased transfers of ownership and so on. The chairman warned developers the agency was prepared to take court action to force debtors to work with it.
Finance Minister Sammy Wilson has warned against a firesale of assets which could flood the local market and depress prices.