Revenue NSW holds payments, refunds, unpresented cheques, dividends and other unclaimed money when people move, die, change their name or lose relevant paperwork.
The department contacts people to return their funds, last year sending more than 11,000 letters for unclaimed money worth $8 million.
But only around a quarter of that was ultimately returned.
NSW Finance Minister Courtney Houssos is encouraging people to search the unclaimed money register.
Courtney Houssos is encouraging people to go online to see if they are owed money. (Dean Lewins/AAP PHOTOS)
"With $230 million still available in unclaimed money for people in NSW, I encourage everyone to get online, search the register, and get the money that's owing to you," she said.
A further $343 million is available to claim for people outside of NSW or for whom there are no available details.
The average amount of unclaimed money owed is $391 and the revenue office has returned more than $150 million over the past decade.
Still, more money was being referred than was being claimed, state revenue chief commissioner Scott Johnston said.
"We want to make sure everyone knows about the unclaimed money register, so they can jump online, find out if any money is owed to them and undertake the process to get it back," commissioner Johnston said in a statement.
"That way we can ensure more money is being returned to those it belongs to, rather than sitting with us for extended periods of time after enterprises and organisations pass it on."
The register is free to search, and Revenue NSW sends the money within 28 days of verifying an applicant's supporting documents.
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