Customs officials have confirmed they have been directed by the immigration minister, Scott Morrison, to describe asylum seekers arriving by boat as “illegal”.

The Customs chief, Mike Pezzullo, told the Senate’s legal and constitutional affairs committee on Tuesday that invoking the term “illegal” to describe boat arrivals was not an initiative of the officials charged with immigration and border protection.

“We didn’t generate it within the service,” Pezzullo said. “It is a term that changed after the election.”

Labor senators used a Senate estimates hearing on Tuesday to pepper officials with questions about the change of terminology since the September election.

The previously sanctioned means of referring to asylum seekers was “unauthorised boat arrivals” under John Howard’s government, and “irregular maritime arrivals” under Labor.

Customs officials reported on Tuesday that the official term enshrined in Australia’s migration legislation was actually “unauthorised maritime arrivals”.

The newly elected government had, however, issued a directive that henceforth, officials call boat arrivals “illegal”.

Labor senators inquired whether any convictions had been obtained, given the activity was now being described by government and its representatives as “illegal”. Officials told the hearing there was no offence under the Migration Act for entering Australia without a visa – hence the lack of convictions.

The minister at the table, the Liberal senator Michaelia Cash, deadbatted that question about convictions, and a further question about whether the government intended to legislate to make arriving by boat actually illegal, as opposed to rhetorically illegal.

Cash detailed instances when senior Labor figures, including Kevin Rudd and Julia Gillard, used the term “illegal” in the context of unauthorised maritime arrivals.

The term illegal had “been used by former Labor prime ministers”, Cash said on Tuesday. She noted “various terms” describing boat arrivals had been invoked by various governments over decades.

“This government has made the decision to use the term illegal,” she said.

Cash said, despite declaring the activity in a generalised sense illegal, it was in fact “not illegal” to claim asylum.

“I confirm for the committee’s benefit, it is not illegal to make a claim for asylum. And indeed the Coalition government has never suggested that it was.

“It is however, as is the evidence that has been given to the committee, illegal to enter Australia without a valid visa, and therefore the use of the term illegal maritime arrival refers specifically to that person’s mode of entry.”