This topic can be hotly debated and there are many many differing opinions and answers offered.

Let me start this with a disclaimer that you should not take any of this information as legal advice nor specific advice to any particular situation you may find yourself in. This information is pulled together from many different resources, referenced where possible which may aid you in finding the relevant official group who can give advice.

Servicing
While it’s not the intent to cover servicing in this thread, it is worth mentioning at least in passing.
The Trade Practices Act prevents the practice of exclusive dealing by attaching conditions to the sale of goods that restrict the buyers freedom of choice to deal with whom or in what they choose.
Essentially this means that a vehicle manufacturer cannot specify that you must have your car serviced by a particular dealer or group of dealers as part of the warranty conditions.
Nor can a manufacturer void a warranty simply because a vehicle owner has chosen to have a vehicle serviced outside the dealer network.
A vehicle manufacturer can however refuse a warranty claim where there is evidence of lack of maintenance, inappropriate work practices or where a non-genuine replacement part has failed or caused other damage.
Source: Compare Dealer and Non-dealer Car servicing | Motoring | RACQ
So as long as you use a licensed repairer and they use appropriate parts and fluids, oil for example must meet VW’s 504.00/507.00 standard, you shouldn’t have a problem with warranty.
A great reference is here: Volkswagen Oil Standards – lists of all approved oils – Skoda – Audi – Seat | my-gti.com

With Volkswagen’s (and Audi’s and Skoda’s) it’s highly recommended that you choose a specialist rather than your local service station, but in either case you should be fine.

NOTE: If you do use someone other than a dealer (and even if you do use the dealer network) it can be well worth keeping all your receipts proving the work was done at the appropriate times AND what fluids/parts were used. There have been reports of dealers requesting evidence that serving was done prior to approving warranty claims, something which is not unreasonable.


Modifications
In simple terms modifications do not automatically void the entirety of your warranty. Essentially, if there is a failure that occurs only as a result of what you have done to the vehicle, it would not be covered by warranty. If the failure would have occurred anyway, it is covered by warranty… and here is where it starts to get more complex.

Any modifications give the manufacturer a possible “out” against your warranty claim if they can reasonably attribute any failure to your modifications. If the modification has nothing to do with the defect then they have no grounds to invalidate the warranty.

For example if you were to put a bigger turbo in the car and tune it to within in inch of it’s life and the power window switch fails at 3 months old it should still be covered by warranty. But if one of the pistons lets go it would be very easy to for the dealer to point at the performance improvements as the root cause.

While it has been said it’s up to the manufacturer to prove it was the fault of the modification rather than a faulty part, the reality is if they make that decision it is then up to you to prove it really was their faulty part and not your modifications, something which isn't a simple task.


ECU Tuning
As above, ECU tuning does not automatically void your warranty, but it does give the dealer something to point at and blame for the fault. Again, if it's that window switch that fails you are still covered.


All else aside, by misrepresenting the nature and extent of both the dealer voluntary warranty and the statutory warranty, the dealer and its representatives are arguably in breach of civil and criminal provisions of the Competition and Consumer Act. These breaches involve substantial penalties for both corporate bodies (and individuals ($220,000 per offence) involved.


Government Bodies:

Australian Automotive Aftermarket Association
Australian Automotive Aftermarket Association

Australian Competition & Consumer Commission
ACCC Home

NSW Department of Fair Trading
NSW Fair Trading: Homepage
Repairs and maintenance - NSW Fair Trading

Consumer Affairs Victoria
Consumer Affairs Victoria - Consumer Affairs Victoria



NOTES: I will update this post with any new relevant information as it comes to hand. If you have a link or something you would like to have included, please let me know.