I have a T430s, with a 16gb of RAM and non-lowpower core i7 it's still going strong and more than adequate for my use. but the original battery only lasts about 20 seconds.
In past I have had hit and miss (mostly miss) with buying third party and claimed "original" batteries from ebay/amazon. Best I've had is around 70% capacity of the original.
Has anybody had a good experience with a specific seller/ brand of batteries to get near original performance?
At this point I would even consider the extra expense that but where could I buy a legit battery for ThinkPad T430s in Australia? Lenovo doesn't seem to sell them.
Personally I consider any 3rd party battery to be a fire hazard, you simply don't know the safety standard of 3rd party batteries and they are the first to cause problems.
Generally you can get a genuine battery online.
We use daily https://www.betterbatt.com.au/
For a HP laptop, we oversized also and has been working well for 6 months.
We've used betterbatt.com.au regularly and never had trouble. Often for older laptops you can't get from manufacturer directly.
Descending order of priority:
1 Directly from manufacturer
2. www.betterbatt.com.au
3. eBay as an absolute last resort
Wow, these are expensive offers, I seem to remember the original Lenovo battery being around $50 3 years ago.
The only good experience I had was buying original batteries directly from the manufacturer, in your case Lenovo. The batteries will be more expensive, but they will last years and are risk free.