The SFO alleges some $3 million was lost, while Maher argues it was less than $1m.
Judge Gibson said if the fraud totalled in the millions as the SFO claimed then Maher was facing a custodial sentence.
The SFO said Maher, FBL's sole director, provided clients with forged documents which deliberately misrepresented the status of the transactions he due to carry out.
He then used the forged documents to disguise that FBL was in financial difficulty, the SFO said in a statement today.
Acting director of the SFO, Rajesh Chhana, said Maher "risked client funds and New Zealand's reputation as a safe place to do business through his arrogant and, ultimately, criminal actions".
"He used his clients' money to maintain the facade of a successful business. Significant losses to client funds could have been avoided if Mr Maher had behaved honestly and accepted that his business had failed," Chhana said.
The SFO began investigating FBL in 2017 after it collapsed and $500,000 in bullion was found in an office safe and creditors were reportedly owed $15m.
Maher had operated the foreign exchange broking business from 1995 until 2017, with shares of the company being held by him and his wife.
The company, founded in 1995, was placed into liquidation in April 2017 and Maher was declared bankrupt in November 2018.
A liquidators' report for FBL, prepared by Christopher McCullagh and Stephen Lawrence of PKF, claimed Maher breached his duties under the Companies Act by continuing to trade for four years while incurring "significant losses".
Liquidators said they considered Maher was personally liable for some debts of the company and had served him a demand for $4m - but recovery was unlikely.
"Maher responded advising that he did not have the means to settle the demand and therefore intended to declare himself as bankrupt shortly," the report said.
Maher had earlier told liquidators he blamed the failure on "too many 'out of money' contracts" and competition that forced him to adopt an overly-large position.
"I had tried to devise a plan to start returning investor funds and start scaling back the company until I could close it down, but I ran out of time and cash flow," Maher said.
The website for FBL claimed Maher had "over 20 years experience in the foreign currency industry" and "an extremely high level of business integrity and customer service".
Several of the company's clients have told the Herald payment delays with FBL seemed to accelerate from late 2016, while Maher also offered high-risk foreign exchange investment services.
