The charges related to seven victims, who lost amounts ranging from $137,500 to $10,000. The total amount equalled $351,300.
Crown prosecutor Jacinda Foster told the court that due to things moving rapidly this morning, she would provide a summary of facts to the judge on 5 May.
She also told the judge she would withdraw the remaining charges once the guilty pleas had been entered. Duffell was to go to trial on 31 charges.
Judge Kim Saunders remanded Duffell on bail. He will appear for sentencing on 21 July.
The charges against Duffell were first laid in September 2014, but had been delayed after two earlier trial dates in February 2016 and July 2016 were cancelled due to a change in lawyers.