Good morning. I'm XXXXX XXXXX, a/k/a "DrKlahn."
My experience with low-end commercial UPS is that they have about a five year lifetime, and there's not much fix to them. The manufacturers are reluctant to provide service manuals or schematics, and service generally consists of "If you mail it to us, we'll swap it for a new one that works." This is a good deal for the UPS manufacturers, because few buyers want to ship a heavy UPS back to the manufacturer.
I quit recommending new-in-the-box UPS units to my customers several years ago. I now buy used APC commercial units. They're not expensive, they're fixable, they use standard batteries, and they're made to a better standard than "consumer" UPS units.
The reason budget "consumer" UPS units aren't fixable is that it's economically impractical. This is not surprising, considering what it costs to repair electronics today. In most repair facilities there's a two hour shop time minimum charge, with technician time at $US 100/hour. Then comes the cost of parts - figure a $US 50 minimum for parts. The minimum cost of a repair is thus about $US 250. Then add another $US 40 for shipping if the repair must be sent off to be fixed at a regional repair center outside your local area.
The rule of thumb for repair vs. replace is repair only when the repair cost is less than 2/3 of the cost of replacement. Therefore, if you can buy a replacement for $US 350 or less, you're better off with a replacement.
There may be an escape hatch on the warranty, however. If you purchased the unit with a credit card, many of these cards double the manufacturer's warranty up to one additional year. If this might apply, check your cardholder's benefits flyer. If this does apply, contact your card supplier at the number in the benefits flyer to have the unit replaced or refunded.
Otherwise, it's far more sensible to replace the unit. Brand new "consumer" grade 400 VA UPS units are selling for about $US 40, though I would recommend (and I use) a 1 KVA unit. 400 VA doesn't last very long when trying to run a computer and monitor, and unless the monitor is plugged into the UPS there's no way to shut the system down cleanly.
The APC Back-UPS 650 would be a good choice. It doesn't seem to be as widely available in Australia as here in the US, but it does show up on ebay .AU at reasonable prices - see the link below. The Smart-UPS 1000 is ideal, but perhaps a bit expensive.
In general, any of the APC Back-UPS or Smart-UPS units will give long and reliable service, and when it comes time to replace the batteries they won't break the bank.
So, in summary: There's not much fix to a budget consumer UPS. Repair is prohibitively expensive. Replacement makes sense economically, but I recommend buying an APC next time around because they're reliable, repairable, and use standard batteries.