It is the same between US and Canada (slightly different rules in each direction)...
.. and has been at time for internal but interjurisdictional between
US and between
Aus States/Territories, when there has been a perceived phytosanitary need in one to protect its produce from something known to be flaring or endemic in another. These rules can be enforced by State Troopers with State agricultural officers at roadblocks on major highways. At these, in US at least, to stop, to interrogate and potentially to search vehicle and even persons are allowed. We experienced one of these when travelling as a family from Nevada into California. All very quick and courteous (quick conversation, explanatory leaflet given), but clearly seriously meant. I don't know if the Maryland tags on the car and trailer, or maybe the fact of the camping trailer suggesting prior or intended visits to campsites in agricultural areas, were the reason for being selected for the stop, to which not everyone was being subjected.
In the US there are also State laws forbidding the personal imports from another State of items legal in the latter, including fireworks and firearms. While 'arbitrary' searches of border crossers for such is unConstitutional, those stopped and searched for any other reason may face State charges if such items are found.
At the moment, of course, internal travel of persons between some States/Provinces/Territories both of Australia and of Canada is forbidden by the human health and quarantine rules of the potential receivers. This would be unConstitutional in the US, I assume.
(mere guide at) Jet Age Museum, Gloucestershire Airporthttp://www.jetagemuseum.org/
TripAdvisor Excellence Award 2015http://www.tripadvisor.co.uk/Attraction ... gland.html