When you check it in - Just tell them its a surfboard, If you mention windsurfer they still think in terms of a Wally.
They probably not even give it a second look - weigh it, tag it, and send you to the oversize drop off - Get to the airport early and be in queue before the counters open.
If your really worried - have a mate come with you (with gear, you obviously will need to have someone drop you off anyway) to the airport to ready to take your gear home if it all goes pear shape.
I've taken a windsurfer (about that size) on Air NZ to Fiji without a worry - Fiji Air should be even more relaxed.
Lets us know how it went - and enjoy