Four Brumbies players, two coaches and some staff have been diagnosed with the mumps despite all being immunised against the disease. Two more players have been quarantined as a precaution, but have not been diagnosed with the mumps.

Initial tests were inconclusive, but results this week diagnosed mumps as the mystery illness sweeping through the team.

Irae Simone, Bayley Kuenzle, Darcy Swain, Mack Hansen and attack coach Peter Hewat were told to stay at home last weekend and missed the Brumbies' heartbreaking loss to the Highlanders.

Since then players have been given booster shots and anyone showing signs of illness have been told to go home.

Every player flying to New Zealand on Thursday had a blood test and will be free to travel, but only allowed to board the plane if they are "completely well". The Brumbies will likely take extra players to Hamilton in case of further illness abroad.

Simone, Kuenzle, Hewat and McKellar have returned to training and have been cleared to fly to New Zealand. It's understood health officials have deemed there was only a low risk to crowd members at the Brumbies-Highlanders match last weekend and McKellar praised officials for moving quickly.

"All staff and players have been vaccinated so we're not sure where it's come from or how it got into the group," McKellar said.

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"Since the first symptoms appeared, as a club we did everything we could to get on top of it.

"I missed my first session in seven years last week, I had to isolate myself for a couple of days. Peter Hewat was the same and wasn't available on game day, it wasn't just the players."

The Brumbies have been working with Rugby Australia, SANZAAR and ACT Health after finding out about the diagnosis.

The Brumbies' gym and University of Canberra headquarters was sterilised over the weekend and players have been extra vigilant with cleanliness.

Brumbies chief executive Phil Thomson says the game against the Chiefs was never in danger of being cancelled after medical staff moved to isolate ill players.

Mumps is a viral infection that affects the salivary glands. Some players and staff were vaccinated but were still infected.

Rugby Australia has also been working with the Brumbies following testing this week, implementing several steps to protect players, their families and officials.

The Brumbies contacted opposition clubs to inform them and players will be checked at the airport before departure to New Zealand.

Rugby Australia's steps include: Isolating suspected and confirmed players for five days, ensuring Brumbies players, staff, partners and children have immunisations up to date, recommending other Super Rugby franchises have up to date immunisations, allowing only completely well players and staff to travel to Hamilton, preventing players with symptoms from boarding a flight in Canberra and isolating anyone who is ill in New Zealand.

"We've been working with the Rugby Australia chief medical officer and ACT Public Health to come up with measures to try to stop any further spread," Thomson said. "Internally we've quarantined the players, made sure they've had immunisations and given booster shots.

"We were in contact with public health to do everything we could and notified all the relevant people. With international travel, we wanted to make sure we weren't putting other people in jeopardy.

"The risk for fans was extremely low. The main way of getting it is through saliva and coughing. The contact with fans has been very limited. There has only been one player since the game last week who has presented with illness.

"Even it before it was diagnosed, we did everything we could with hygiene and had our headquarters sterilised. There's a real focus with everyone, players and staff, to have everyone aware."

The chaos has put another hurdle in front of the Brumbies as they chase their first win in New Zealand since 2014 and their first against the Chiefs on the road since 2007.

But McKellar believes the players will thrive with their backs against the wall.

"That's the hardest thing about [losing to the Highlanders last week] because we were 15 seconds away from one of the great wins," McKellar said.

"We had our backs to the wall as a club. Blokes playing out of position, illness on game day ... It said a lot about the Brumbies and resilience that this organisation has."

The illness can be spread by coughing and sneezing or touching a contaminated surface.

It can lead to inflammation of the brain, inflammation of the lining of the brain and spinal cord, inflammation of the heart and infertility.

"We're just trying to stay away from [each other]," joked flanker Rob Valetini. "There's hand sanitiser everywhere. We're not [in the gym] with each other any more.

"Dan touched on it. We're a squad of 37, not 23. We're going to back whoever steps up. Resilience is one of our key points and that will be the case this week."