According to Official Guinness Records,
The longest wheelbarrow push was one of some 14,500 km (9,000 miles), from 24 April 1975 to 6 May 1978 by Bob Hanley (Australia), starting and finishing at Sydney and pushing through Townsville, Mt Isa, Tennant Creek, Alice Springs, the Nullarbor Plain, Perth, Adelaide and Melbourne.
For a complete list of 1977 records, please visit 1977 Guinness Records in Australia.