Your tender documents should include a form to be completed by the tenderers with their submission (and especially before signing a contract) listing the costs of all tradespeople to be used on and off the site.
This list can be used to assist in fairly reviewing variations and ‘hourly rates’ work.
This list should consider Normal working hours as well as Overtime hours, as well as any other specific peculiarities that may apply to your site.
… And do be sure the GST is noted clearly; as either included or excluded.
And also make sure ‘Profit & Overheads’ are also clearly noted to avoid confusion.