"It's a big game for both teams because we've both lost a few on the trot so it's must win for us," Arnold said on Tuesday.
"We've got a tour to South Africa after this and it's never easy to win there."
Arnold said that despite falling to a 21-8 loss against the competition pace-setting Crusaders in their last outing, his team's performance had given them confidence they could keep their season alive.
The Brumbies had a great start and plenty of chances in the second half but failed to capitalise.
"The Crusaders are top of the log and we stuck it right to them and probably should have won if we managed that last 20 minutes a bit better," he said.
"We've been in games all year and competed with some of the better teams so there's still belief we can go deep into the competition."
As well as Super Rugby seasons at stake, players from both teams can make a claim for a Wallabies jersey for next month's Test series against Ireland with a strong showing at Canberra Stadium.
One position where there's plenty of heat is second row, with Brumbies locks Arnold, Blake Enever and Sam Carter duelling with Rebels duo Adam Coleman and Matt Philip.
Arnold said it was the perfect stage to show Wallabies coach Michael Cheika what they had to offer.
"There's battles everywhere and it's a good chance for players to put their hands up for higher honours," he said.
Arnold said the team felt fresh coming off the bye and thought he was starting to find some form after a string of injuries.
"It's good to get some good minutes under my belt so hopefully I can stay injury free and keep building," he said.