"It is disappointing it had to end this way - very sad. It's coming up seven years with Alan, but I have to keep moving forward."
The runner, who turns 19 this month, heads to Europe shortly for his final preparation before the world juniors in Barcelona from July 10-15. He is part of a 12-strong New Zealand party.
"I fly to Auckland next Friday where I have an 800m race arranged to gauge where I'm at and then fly out to the worlds the following Sunday," Mathas said.
"I'm pretty happy with my form and think I've improved, especially over the last two weeks."
Determined and ambitious, Mathas' target is an Olympic gold medal and his "dream goal" this year was to make the London Olympics, though that always seemed a pretty long shot.
The NZ secondary schools 800m champion for the past two years, Mathas has a best time for the distance of 1:49.57secs which puts him second on the national rankings behind Olympic 1500m silver medallist Nick Willis.
He notched up that time at the Porritt Classic meeting in February and believed he was capable of running 1:47, but has not been able to bring his personal best down in the subsequent months to put him in Olympic contention.
"I have been talking to High Performance and they've said to me I must think about where I want to go and what I want to achieve and then when I come back from the worlds we can find a new coach," Mathas said.
Rubick said the split was almost inevitable, but wished his former charge well in his future endeavours.
"It did get a bit ugly, but hopefully now we can leave on somewhat friendly terms," Rubick said yesterday.
"I think he burned bridges when he perhaps could have discussed redefining our relationship, especially at such an important time a few weeks out from the world juniors. He's a pretty motivated guy and that has helped in his development, although when he sets his sights on something he wants it now. I would have preferred he aimed for the Commonwealth Games first and then maybe the Olympics," Rubick said.