The administration yesterday unveiled its plan to keep New York City solvent after June 30, which features a set of standby federal quarantees of up to $2 billion in long-term federal securities.

But the program, which has not yet been formally sent to Congress as proposed legislation, does not contain any federal assistance for the city's seasonal financing requirements, something most New York officials say is needed, too.

Treasury Secretary W. Michael Blumenthal, testifying before a House Banking Subcommittee, said that the administration's assistance is contingent upon the city adopting a budget that is in balance by 1982.

Blumenthal said that New York City -- which has been receiving seasonal financing assistance from the Treasury since 1975 -- needs continued federal help because it still does not have access to normal borrowing channels.

The Senate Banking Committee, led by Chairman William Proxmire (D-Wis.), unanimously concluded last month that New York City's financing needs could be met by combined city-state action and that no federal involvement is needed.

The House Banking Committee is more receptive to continuing aid to New York City when its seasonal borrowing authority runs out June 30. But Chairman Henry S. Reuss (D-Wis.) said he would not try to push a bill through the House unless there is a change of sentiment on the Senate side.

Proxmire said in a statement yesterday that he still does not think federal help is needed, but will hold hearings when the administration sends a bill to Congress.

Many of the pieces of the administration's proposal are not yet in final form, Blumenthal said. One of the missing parts is the exact nature of the federal loan guarantees, which will be given "only to the extent that public markets and private leaders do not provide the necessary funds on an unguaranteed basis."

The $2 billion in guarantees would be spread over the next four years and would cover less than half the $4.5 billion in long-term borrowing the Treasury thinks the city must do between June 30, 1978 and June 30, 1972.

New York Mayor Edward Koch and Felix Robatyn, head of the state corporation set up in 1975 to do the borrowing the city could no longer do on its own, developed a plan last month that included $2.5 billion of federal guarantees on a long-term borrowing need of $5.1 billion.

Rohatyn said yesterday that while the Treasury's plan is a "good starting point," it has "some fairly serious flaws."

Rohatyn said the city cannot do the level of seasonal financing on its own that the plan envisages, that the overall size of federal help is too small and that New York needs permanent, not standby guarantees.

But Rohatyn -- as well as New York Gov. Hugh L. Carey and Mayor Koch --ment was the administration's commitment to help the city, not the actual details of the plan.

President Carter pledged yesterday to work hard to get a New York City financing package through Congress.