Only 50% of owners voted for a levy increase — is a quorum of 80% required for the AGM to proceed?

No, to achieve a quorum at an AGM, Section 77 of the Owners Corporations Act 2006 states that at least 50% of the total number of lots or 50% of the total lot entitlement must be present in person or by proxy. You have stated that 50% of owners voted for the increase, so I will assume that a quorum was present at the AGM.

As the setting of annual fees is done at the AGM, the decision to increase the yearly fees from $2,000 per annum to $3,000 per annum could unlikely be challenged if a quorum has been achieved with at least half of the owners based on lot entitlements present at the meeting. In addition, if a majority of those present at the meeting voted in favour of the increase, in my view, the owners corporation has approved the fee increase.