I'd say it depends what requirement you've got to be in WLG the day you plan to land. If you can let it slip a day or so you should be ok (assuming you're willing to tolerate the cost of a last minute flight and possibly a hotel in SYD.

April has a fair bit less chance of thunderstorms than February for Sydney, the weather is generally more mild (and often wet) but less turbulent -- though these days weather patterns are something that we can rely on less and less.

My general view is that I'm willing to take a risk on a flight of a few hours worth up to a couple of hundred pounds, but not one that's significantly complex/long and worth thousands on the day; so personally I'd probably do something similar. Right now you can buy a flight from SYD-WLG for tomorrow for £230-ish. It's also possible there might be some good last minute avios availability which could reduce the risk further.

You'll still need to prove you've got the right to enter Australia for boarding BA15, but in theory I think you could use the airside transit at SYD if you're HBO?