Invoice amounts varied from $3403 to $30,000. The frequency of fake invoices increased with time, spiking in the month before last Christmas, when $140,000 was stolen.
Catterall told police she did it "for money, greed and to fund a methamphetamine habit."
Catterall's lawyer Marcus Edgar sought name suppression as publishing her name would hurt her family members, including elderly relatives, and those unaware of the charges.
Judge Eddie Paul refused suppression and encouraged Catterall to tell relatives what happened.
Sovereign did not say when Catterall was sacked but said policyholders' money was safe.
"The fraudulent activity... was an isolated incident and did not involve customer or adviser funds," a spokeswoman said. "There will be no impact on Sovereign's financial strength rating or our customers. We have taken the issue very seriously - operating with integrity is fundamental to our business and the financial services industry."
Sovereign was seeking to recover the money.
Catterall will be sentenced next month.
Australian Symon Brewis-Westontook over as Sovereign chief executive last year. In 2011, Brewis-Weston married newsreader Sandra Sully.
Catterall, also known as Mechelle Arahanga, was declared bankrupt on April 7. In the bankruptcy notice, she was listed as a sickness beneficiary, living in Papakura. She could not be reached for comment.