The Heart-Mariners clash was the only A-League match on Sunday and the temperature in Melbourne topped 33 degrees just after kick-off.

Despite the extreme conditions, both sides produced an entertaining affair with Heart striker Eli Babalj's goal in the 50th minute proving the difference.

Arnold didn't use the heat as an excuse, but was left confused as to why the game was played in the heat of the day rather than at night time.

"I don't understand why the game had to be played at 5pm today," he said after the match.

"I think when we were down here last time we talked about the good spectacle that our game was against Melbourne Victory and I truly believe that the level of A-League has risen substantially, especially with the night games.

"The day games are very very hard for the players, they were extreme conditions out there, very tough. You saw the tempo sometimes drop, if it's a night game and all the games are played at night – we're the only game today so it would have been nice to kick off at 7 o'clock in better conditions.

"You'd like the games played at night time. Every game you watch at night time the spectacle is so much better, the players have so much more energy and the intensity is so much quicker rather than day time kick offs – 5 o'clock in the middle of summer is a bit hard."

On general play the Mariners probably deserved at least a point and will be left to rue a penalty miss shortly after Babalj's strike.

English debutant John Sutton stepped up to take the penalty but his shot hit the post and bounced out to Patrick Zwaanswijk, who scuffed his attempt with the goal at his mercy.

Arnold was left bemused as to why Zwaanswijk didn't take the penalty himself.

"I'll ask that question behind closed doors because Patrick is normally our penalty taker," he said.

"I haven't seen him take a penalty so I was surprised. At the end of the day I'm a firm believer in whoever feels confident should take the penalty so John was confident enough to step and take it and you need to be brave to do that."

After going 15 games without losing, the Mariners have now lost three times in the past five matches and have seen their lead over Brisbane at the top cut to just three points.

But Arnold sees no reason to panic with his side already achieving their pre-season goal of making the top six.

"I think it's about the performance and if you want to look at the big picture that's three losses in 19 games," he said.

"If you look at the big picture it's still a great performance and as I said to the players after the game 'let's just move on with it'.

"When you're playing well, everyone's fit and healthy and let's enjoy the next couple of months together and forget the table.

"We're in a fantastic position for top two and our goal at the start of the year was top six so we've achieved our goal and let's just enjoy it."