Voluntary administration, according to the Australian Securities and Investments Commission (ASIC), is a process "designed to resolve a company's future direction quickly".
"An independent and suitably qualified person (the voluntary administrator) takes full control of the company to try to work out a way to save either the company or its business," ASIC says on its website.
Sumner said in a statement to clients and other stakeholders that "Mossgreen has chosen to take a path of voluntary administration during the month of January at a time that will least impact our clients and which will allow the company to restructure its business".
"The company is looking forward to a very strong calendar of auction-sales that are contracted and already catalogued for the first half of 2018, starting in February," he said in the statement.
"No vendors will lose any money in this process and neither will any of our staff, who will also be fully supported through this process."
A creditors' meeting is due to be held in early January in Sydney.
Contacted by the Herald this week, Sumner said the administration wouldn't impact the New Zealand business.
"This does not affect the NZ business and it is a different entity thus Mossgreen-Webb's is not in voluntary administration," he said in an email.
Set up in the 1970s, Webb's has exhibited some of the country's most well-known artists, including works by Colin McCahon and Gottfried Lindauer.