First off to be precise, this was a ”proposal to recognise Aboriginal people in its constitution and establish a body to advise parliament on Indigenous issues".
Some examples of what I think were sadly effective for the no campaign:
"This will allow indigenous peoples to reclaim your land"
"It will only further divide our nation"
"We don't know how this might be misused"
These all play on peoples fear. On the other hand some indigenous peoples also were campaigning for a no vote, primarily because they thought it wasn't strong enough.
This gave voters a lot of reasons to hide behind while voting no.
And all this was not helped by a rather poor yes campaign that barely did anything to address misconceptions.
Probably more than half, but there's actual decent content on there too.