It sound like you have tripped over either a rort or, just maybe, an accounting and invoice error.
I think in your position, I would submit a motion to the next strata committee meeting, asking why there are invoices that seem to suggest that the secretary has used OC funds on their own property and give the person concerned the opportunity to go on the record to say that this has never happened and explain the mistake.
If they fail to do so then you put the same motion to a general meeting (where it must be put on the agenda). This time I would add “give reason why these funds should not be remitted back to the OC.”
If that fails, it’s off to NCAT with whatever charge or claim you think is most likely to stick.