Thank you for your pm. I'll reply here as others may be following the information found.
You asked about Hannah in census. There are 5 Hannah Dawsons in Durham in 1841, 4 with their families and 1 a female servant in Ryton, Durham, but it's not possible to know if that's her, she may have married by then, she could have moved away from the area, or died
On freebmd there are two possible marriages in the Weardale registration district which covers Tow Law where the family were in 1851 More likely is this one in the Durham &c district which covers Medomsley and Lanchester where the family were in 1841.
Dec 1840 Durham &c. 24 93 DAWSON Hannah
names on the same page are
John GIBSON George HERRON George LAWSON and William TOWNS
Couldn't see them as a couple in 1841/51 census though.
On the NZ board Lucy2 gave a link to the Victoria outward passenger lists, have you checked for the rest of the family?http://proarchives.imagineering.com.au/index_search.asp?searchid=42