The third-placed Mariners were impressive in their showing against the Roar, taking the lead on two occasions after going down early, and Arnold felt it was an opportunity to end Brisbane's unbeaten run of 20 matches.
"It was a fantastic game, but I thought we were the better side and should have gone on to win it," said Arnold.
"Two dead moments killed us tonight which is better to happen tonight than in a semi (final). It was disappointing to concede from a corner and then a really soft throw in.
"Overall though I think our performance was fantastic. When the boys are sitting in the dressing room disappointed after a 3-all draw with the team leading the competition it shows how far we've come."
Mariners skipper Alex Wilkinson agreed with Arnold's sentiments but was already looking forward to their next outing against Newcastle Jets this weekend.
"We're gutted at the moment to be honest, disappointed that we didn't pick up the three points, but as Arnie said we can be proud of our performance. I thought the boys in midfield and upfront put in an enormous shift, especially backing up after the break," said Wilkinson.
"We will gain confidence out of our performance, and I think we've learned that we can compete with anyone in the competition.
"The best thing for us to do now is put that behind us and look forward to the game against Newcastle on Sunday."