I am a UK Citizen on a working holiday visa in New Zealand but want to go on holiday to Australia, I looked at evisitor visa (subclass 651) and it says if you hold another visa it can replace it. Is that applicable to ANY visa or ones held in Australia?! Im so confused and dont want to apply for the evisitor visa incase my WHV for New Zealand where I am living is null and voided....
Australia and New Zealand (which I assume you are aware, but I'm stating it just to be clear) are different countries.
As a result, any visa you get in Australia is separate, distinct, and does not affect a visa you received in New Zealand.
Any visa you receive from Australia does not affect visas from other countries, including your working holiday visa from New Zealand.
If you held another Australian visa of sorts, you might have a different issue, but as you don't, this does not affect you.
Enjoy your visit to Australia, but be aware, it doesn't "pause" your NZ visa in any way, and it continues to tick away while you're out of the country.