Keep in mind that very soonnnnn (fingers crossed) the passenger lists will be searchable
so far 17% searchable at LDS pilot site
http://pilot.familysearch.org/recordsearch/start.html#c=fs%3A1609792&p=collectionDetails
Or you could post a request asking a Wellington based rootschatter to check the card index at Wellington Archives. It's called "The Index to Government Assisted Immigrants, 1871-1888 Card Index". It's lists pretty much the same people as the LDS Pilot for that time period. Basically it's a whole lot of white flip cards arranged by passenger surname which may (or may not) include your people.
Or if you have easy access to a LDS Family History Centre you can order a microfilm of the card index. You'd need to order...
"VAULT - BRITISH Film 1514962 containing Webb, Sarah Ann to Zyglewski, Friedrich"
For what it's worth, there is a Mr, Mrs and Child Winter arriving in Auckland on the Myrtle on Feb 16 1880. The only problem is it came from Tonga (according to PapersPast). Maybe you could ask an Auckland based rootschatter to see if Auckland Library has any further info on the passenger manifest (although I think that would be unlikely)
Here's the search engine link:
http://www.aucklandcity.govt.nz/dbtw-wpd/passengers/passenger.htmlRegards
Beg