It depends on whether "person" means "owner"
If Bob is liable, it's not under the Impounding Act of 1955. In that Act, the occupier of land is allowed, but not required to impound trespassing animals. This is made clear in s 21 of the Act, which says "the occupier...may seize and impound any stock trespassing on the land." A quick search finds no sections of the Act requiring an occupier to impound trespassing cattle. So it seems Bob is free to send the cattle on their way, at least under the Impounding Act.
However, liability for cattle and cars is also covered by the "Animals Law Reform Act of 1989." The two subsections of "Section 5" of that Act appear to broaden the class of people who could be held liable for damage “caused by an animal straying onto a highway.” Neither subsection explicitly mentions the owner. Instead, both talk about the "person" who is liable. The first, s 5(1) says the part of the common law that “excludes or restricts” “the duty that a person might owe to others to take reasonable care” to prevent damage no longer applies in New Zealand. The second, s 5(2), says a court must determine "whether a person is liable...for damage caused by an animal straying onto a particular highway..."
Given that Impounding Act explicitly says "owner" not "person," common sense suggests the use of the word "person" rather than "owner" in the Animals Law Act of 1989 means that Act allows others besides the owner to be held liable for damages. Whether New Zealand courts agree, and whether they have interpreted the “Animal Laws Act” in a way that would include Bob is a matter of fact that can only be answered by someone who knows New Zealand law.
Added: Something fun to read
Law professor Robert Ellickson studied how people actually resolve disputes over wandering cattle in Shasta county in northern California. There's a readable summary of what he found here. (The title of his book, "Order without law," sums up his main finding -- there are rules that are enforced, but those rules have little to do with the formal law or law enforcement.)