Union officials say the companies building three massive gas plants on Curtis Island, off Gladstone in central Queensland, are breaching the agreement over workers' travel times.
The Australian Manufacturing Workers Union (AMWU) says the island ferry service takes too long to reach the mainland after a shift and workers are not paid for the added time.
About 300 union members rallied in Gladstone yesterday.
AMWU spokesman Phil Golby says the matter could be resolved through better planning.
"Gladstone Regional Council approved the two ferry terminals side by side and the road in," he said.
"They've got a single road that comes in and out.
"The company itself, Bechtel - they aren't managing the ferries coming to the terminals.
"They're getting them there at the same time.
"When they're getting here at the same time vehicles are being forced into a bottleneck because it's only a single-lane road." Mr Golby says the situation goes against the terms of the workers' enterprise agreement.
"When the agreement was negotiated, it was negotiated so they had an hour's travel each day, and we had 15 minutes each way on the ferry, which came to one-and-a-half-hours travel each day," he said.
"At this point in time we seem to have guys a lot longer than that and some of them are spending up to 14 hours away from home." Bechtel's Gladstone general manager Kevin Berg says the company aims to make ferry travel times as efficient as possible.
Mr Berg says Bechtel is working to improve transport systems in both directions.