Marae are very common hires for large groups - it's basically like hiring a hall, but with the additional benefit of subtly exposing people to Māori culture. As someone who has attended many marae functions, I stress how subtle it is. Most people don't even take their shoes off (and are not expected to). It's just like going to a hall.
They don't say how many people attended, but realistically $2.5k is couch cushion change for almost any council, and if there were enough people to hire a marae then that is likely to be a tiny amount of money per head compared to the cost of their time.
The weirdest part of this article in my view, is not that the council held a planning day at a marae, but that this guy opposes going because of his Christian faith. The thing is, Māori have historically been very Christian. Many of the common karakia are translations of bible prayers, and many customs have roots in Christianity. He doesn't oppose going to a marae because of his faith, he opposes it because he doesn't want it in another language.
It also doesn't sound like he wants religion out of government, he just wants to make sure it's in the original Christian language: modern New Zealand English.