But a Dunedin landlord expects any legislative change will force landlords to increase rents.
Mr Twyford said an inquiry into boarding houses in New Zealand by the parliamentary social services select committee was presented to Parliament this month.
The inquiry found some boarding house tenants lived in ''unsafe, unsanitary and unhealthy'' conditions.
A licensing regime for boarding houses was supported by the Labour Party, Green Party and New Zealand First.
But the select committee, which had a majority of National MPs, did not recommend licensing.
Mr Twyford said vulnerable people were often left to rot in slum conditions.
If elected, Labour would overhaul legislation to make certain landlords who did not meet standards could be declined licences, ''to ensure a level of decency and safe and sanitary accommodation''.
The legislation would mandate councils to be responsible for enforcement.
Labour would also provide more temporary and supported accommodation.
The policies would be announced during the election campaign.
''We certainly think it needs to be made a priority.''
Dunedin boarding house landlord Ted Ottrey said he expected the costs to administer licensing would be passed on to the good boarding house landlords and rents would rise.
''It's another thing to push prices higher.''
National MP Melissa Lee, the select committee chairwoman, said it did not recommend licensing because local territorial authorities already had provision for a registration system.
Better information sharing between territorial authorities and central agencies would address many of the issues.
Dunedin City Council environmental health team leader Ros MacGill said if boarding houses were as bad as Mr Twyford believed, then legislation was necessary.
''There must be a gap in the system if the conditions are that bad.''
A complaint had to be made about a boarding house for the council to act, Ms MacGill said.
''We don't have anything to do with these houses at the moment, unless somebody unhappy about the conditions rings up to complain, and we would have to investigate it from there.''
