More information would be useful i.e.

When did Thomas MERRETT marry Sarah BENFIELD - no Gloucestershire marriage of this couple - or on the Church of Latter Day Saints.

? When did Thomas first appear in New Zealand and what are the children's names? Sometimes middle names can be family surnames + naming patterns

Your message on Rootsweb, say he came alone from San Francisco in the 1850's to Oz - with a mention of Michinhampton.