In March, Mr Johnston joined the board of Saab Technologies Australia, a subsidiary of the Swedish military contractor, which has been on the receiving end of millions of dollars in government contracts this year.
Mr Johnston is understood to have advised Saab on the government's decision-making process, as Saab sought a multimillion-dollar contract to fit out Australia's fleet of frigates with a new combat management system.
The Herald has been told the job was not the subject of a formal tender, with Mr Johnston helping to advise on the timeframes around government decisions according to a source close to the company. The Department of Defence has not responded to questions on this.
Earlier this month Defence Minister, Senator Marise Payne, announced Saab had been chosen to develop the new system which will be used on all of Australia's future ship projects.
Two days later, Ms Payne made another announcement, with Saab winning a contract to update the control system of the ageing fleet of Collins Class submarines.
As defence minister, Mr Johnston was involved in plans to replace Australia's fleet of submarines.
In November 2014 he poured scorn on the ASC - formerly known as the Australian Submarine Corporation - which he said had blown their budget by as much as $600 million.
"I wouldn't trust them to build a canoe," he said at the time.
The controversial comment forced then-prime minister Tony Abbott to release a statement reaffirming his confidence in the South Australian submarine builder. Four weeks after he made the comment Mr Johnston was replaced as defence minister by Kevin Andrews.
Last month Mr Johnston struck a more conciliatory tone and said the ASC was now "doing world class work".
Mr Johnston was unavailable for comment. A spokesman from Saab said Mr Johnstone had not had any direct engagement on any of Saab's marketing campaigns and had only attended two board meetings since joining the company.
A spokesman for Marise Payne said Mr Johnston had not had any contact with her office since joining Saab. A Defence spokesman said the department is "not aware of any contact made by David Johnston representing SAAB or SAAB's interests".
Independent Tasmanian senator Jacqui Lambie has been pushing for new legislation aimed at disrupting the revolving door between government and the private sector.
She said current restrictions are inadequate.
"It's no coincidence that former Liberal senator David Johnston has a cushy job on the Saab board just two years after he was defence minister," she said.
"Turnbull himself says that when you hold public office, you hold public trust. Why are we letting people profit off that office, and that trust?"
It's not uncommon for military contractors to scoop up senior government figures as advisers, soon after they leave government.
Mr Johnston's former chief of staff, Sean Costello, joined French submarine manufacturer DCNS, now known as Naval Group, in 2015 within months of leaving the defence minister's office.
More recently, former deputy prime minister and defence minister Kim Beazley joined Lockheed Martin's Australia Board in June last year, six months after stepping down as US Ambassador.
The story Former defence minister joins military contractor upgrading naval fleet first appeared on The Sydney Morning Herald.