Trader wants to charge for time and gas for posting?

No it isn't ok. Check out the page below in Trade Me help pages. Let her know it's against trade me T & C's. I imagine they didn't include that extra charge upfront in their auction text either?

https://help.trademe.co.nz/hc/en-us/articles/360007000792-Listing-policies

If they get difficult get TM to help you via Live Chat.

Shipping is not postage alone; it can also include the cost of items needed to safely & securely ship the item. Ie good quality bubble wrap/new cardboard. These things don't grow on trees, so it's fair enough to expect this is passed on. 

And if the seller lives out of town and you've asked them to run into town to post it to you, why shouldn't you contribute to that cost? 

Was it a pick up only item?  Did you get the item for next to nothing? Does it need wrapping to protect it on its journey?   

$10 sounds excessive, but you don't know what hoops the seller has to jump through to ship it to you. Maybe ask nicely and suggest splitting the difference with the extra they're asking for.