So far I have managed to establish that George was in Hokitika from at least March 1866 until December 1867. He doesn't reappear in the Melbourne directories until 1869, which is consistent with him being back in Melbourne by 1868.
There appears to be a second period of absence from Melbourne in the directories, from about 1872 to 1873. He is back in the directories by 1874. I have not discovered where he went, if he went anywhere.
Looking at the births of their children, the sixth child, George Henry, was born in Fitzroy in 1865, and died there in 1866. After young George the next birth involved the twins Alfred John and Charlotte Broadley Griffith, registered in Collingwood in 1867. That would appear to be a bit of a fly in the ointment when George can be shown to be in Hokitika from 1866 to 1867, so the question arises as to how Susan became pregnant. Was she in New Zealand for a time and return? Was someone else the father? It doesn't seem likely that she could have been pregnant before he turned up in Hokitika in March 1866 and have her twins in 1867. Had she been pregnant by March 1866, the twins would have been born before the end of the year. Perhaps George came and went a couple of times?
After the twins there is a substantial gap before the next child was born - Emily Frances, born in Collingwood in 1873, a gap of five to six years. This could have been a natural gap, but the directories do seem to indicate that George was absent from Melbourne in the period 1872 to 1873.
There are no children born or died in New Zealand.
I haven't yet been able to find a shipping record to New Zealand for George, let alone Susan and the children, so anything is possible. However, taking the view that Susan may still have been in Melbourne, but I hadn't spotted her in the directories, I did a special search in the digitised Sands & McDougall directories for Susan Griffith/s, and I was surprised to turn up several references to a Susan Griffiths living at St David Street, Prahran (next to the Windsor Castle Hotel) from 1872 to 1875.
Whether that is my Susan Griffith I have yet discover, but it does introduce another line to examine, particularly if I could discover the children in school records somewhere.